Mark the register crypto API as deprecated

We provide an API to replace bundled cipher implementations in gnutls. However, that API is rarely used (no users in codesearch), has a niche use case (an application replacing the crypto implementation with a faster), but has significant side effects. Any internal re-organization and extension of the cipher API (e.g., #787 (closed)), cannot be implemented without breaking that API.

I propose to deprecate this API in 3.6.x series, and make it a no-op in 3.7.0. This bug is about marking it as deprecated in 3.6.x.

It impacts:

gnutls_crypto_register_cipher
gnutls_crypto_register_aead_cipher
gnutls_crypto_register_mac
gnutls_crypto_register_digest