New function mbedtls_rsa_get_bitlen()

Document, implement and test mbedtls_rsa_get_bitlen().

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/rsa.c

@@ -1014,6 +1014,14 @@ int mbedtls_rsa_get_md_alg(const mbedtls_rsa_context *ctx)
     return ctx->hash_id;
 }
 
+/*
+ * Get length in bits of RSA modulus
+ */
+size_t mbedtls_rsa_get_bitlen(const mbedtls_rsa_context *ctx)
+{
+    return mbedtls_mpi_bitlen(&ctx->N);
+}
+
 /*
  * Get length in bytes of RSA modulus
  */