gh-138252: Add support in SSL module for getting and setting TLS signature algorithms (#138269)

The signature algorithms allowed for certificate-based client authentication or
for the server to complete the TLS handshake can be defined on a SSL context via
`ctx.set_client_sigalgs()` and `ctx.set_server_sigalgs()`.

With OpenSSL 3.4 or later, the list of available TLS algorithms can be retrieved
by `ssl.get_sigalgs()`.

With OpenSSL 3.5 or later, the selected signature algorithms can be retrieved from
SSL sockets via `socket.client_sigalg()` and `socket.server_sigalg()`.

This commit also partially amends 377b78761814e7d848361e642d376881739d5a29
by using `PyUnicode_DecodeFSDefault` instead of `PyUnicode_DecodeASCII` in
`_ssl._SSLContext.get_groups`, so that functions consistently decode strings
obtained from OpenSSL.

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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>