CryptoPkg/TlsLib: replace TlsGetCipherString() with TlsGetCipherMapping()

In the following patches it will be useful if the IANA CipherId lookup
returns a pointer to the whole matching IANA-to-OpenSSL mapping structure,
not just the OpenSSL cipher suite name. Rename TLS_CIPHER_PAIR and
TlsGetCipherString() to TLS_CIPHER_MAPPING and TlsGetCipherMapping()
respectively, and make the function return a pointer to
TLS_CIPHER_MAPPING.

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Qin Long <qin.long@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Cc: Ting Ye <ting.ye@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=915
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Long Qin <qin.long@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>