Add a new X.509 API call for copy-less parsing of CRTs

Context:
The existing API `mbedtls_x509_parse_crt_der()` for parsing DER
encoded X.509 CRTs unconditionally makes creates a copy of the
input buffer in RAM. While this comes at the benefit of easy use,
-- specifically: allowing the user to free or re-use the input
buffer right after the call -- it creates a significant memory
overhead, as the CRT is duplicated in memory (at least temporarily).
This might not be tolerable a resource constrained device.

As a remedy, this commit adds a new X.509 API call

   `mbedtls_x509_parse_crt_der_nocopy()`

which has the same signature as `mbedtls_x509_parse_crt_der()`
and almost the same semantics, with one difference: The input
buffer must persist and be unmodified for the lifetime of the
established instance of `mbedtls_x509_crt`, that is, until
`mbedtls_x509_crt_free()` is called.

include/mbedtls/x509_crt.h

@@ -52,6 +52,8 @@ extern "C" {
  */
 typedef struct mbedtls_x509_crt
 {
+    int own_buffer;                     /**< Indicates if \c raw is owned
+                                         *   by the structure or not.        */
     mbedtls_x509_buf raw;               /**< The raw certificate data (DER). */
     mbedtls_x509_buf tbs;               /**< The raw certificate body (DER). The part that is To Be Signed. */
 
@@ -220,16 +222,58 @@ extern const mbedtls_x509_crt_profile mbedtls_x509_crt_profile_suiteb;
 
 /**
  * \brief          Parse a single DER formatted certificate and add it
- *                 to the chained list.
+ *                 to the end of the provided chained list.
  *
- * \param chain    points to the start of the chain
- * \param buf      buffer holding the certificate DER data
- * \param buflen   size of the buffer
+ * \param chain    The pointer to the start of the CRT chain to attach to.
+ *                 When parsing the first CRT in a chain, this should point
+ *                 to an instance of ::mbedtls_x509_crt initialized through
+ *                 mbedtls_x509_crt_init().
+ * \param buf      The buffer holding the DER encoded certificate.
+ * \param buflen   The size in Bytes of \p buf.
  *
- * \return         0 if successful, or a specific X509 or PEM error code
+ * \note           This function makes an internal copy of the CRT buffer
+ *                 \p buf. In particular, \p buf may be destroyed or reused
+ *                 after this call returns. To avoid duplicating the CRT
+ *                 buffer (at the cost of stricter lifetime constraints),
+ *                 use mbedtls_x509_crt_parse_der_nocopy() instead.
+ *
+ * \return         \c 0 if successful.
+ * \return         A negative error code on failure.
  */
-int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain, const unsigned char *buf,
-                        size_t buflen );
+int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain,
+                                const unsigned char *buf,
+                                size_t buflen );
+
+/**
+ * \brief          Parse a single DER formatted certificate and add it
+ *                 to the end of the provided chained list. This is a
+ *                 variant of mbedtls_x509_crt_parse_der() which takes
+ *                 temporary ownership of the CRT buffer until the CRT
+ *                 is destroyed.
+ *
+ * \param chain    The pointer to the start of the CRT chain to attach to.
+ *                 When parsing the first CRT in a chain, this should point
+ *                 to an instance of ::mbedtls_x509_crt initialized through
+ *                 mbedtls_x509_crt_init().
+ * \param buf      The address of the readable buffer holding the DER encoded
+ *                 certificate to use. On success, this buffer must be
+ *                 retained and not be changed for the liftetime of the
+ *                 CRT chain \p chain, that is, until \p chain is destroyed
+ *                 through a call to mbedtls_x509_crt_free().
+ * \param buflen   The size in Bytes of \p buf.
+ *
+ * \note           This call is functionally equivalent to
+ *                 mbedtls_x509_crt_parse_der(), but it avoids creating a
+ *                 copy of the input buffer at the cost of stronger lifetime
+ *                 constraints. This is useful in constrained environments
+ *                 where duplication of the CRT cannot be tolerated.
+ *
+ * \return         \c 0 if successful.
+ * \return         A negative error code on failure.
+ */
+int mbedtls_x509_crt_parse_der_nocopy( mbedtls_x509_crt *chain,
+                                       const unsigned char *buf,
+                                       size_t buflen );
 
 /**
  * \brief          Parse one DER-encoded or one or more concatenated PEM-encoded