Add EVP_PKEY_supports_digest.

This is intended for TLS client auth with Windows CAPI- and CNG-backed keys
which implement sign over sign_raw and do not support all hash functions. Only
plumbed through RSA for now.

Change-Id: Ica42e7fb026840f817a169da9372dda226f7d6fd
Reviewed-on: https://boringssl-review.googlesource.com/2250
Reviewed-by: Adam Langley <agl@google.com>