Actually add support for SSL_get_server/peer_tmp_key

[samuel40791765]

Issues:

Addresses CryptoAlg-2699

Description of changes:

Ruby has a dependency on SSL_get_server_tmp_key which we've exposed as a no-op in the past. Ruby has a couple of tests that expect the function to actually return a value however, which means we'll have to actually support this.
The relevant information about the server key was saved in a tmp structure in SSL3_STATE (originally ssl3_state_st in OpenSSL). This tmp structure has been removed by Boring, with its contents moved either to SSL_HANDSHAKE or the state machine. peer_key seems to contain the relevant information we want and it's been moved to SSL_HANDSHAKE in this case. The issue is any information in SSL_HANDSHAKE is shed immediately after the connection has been established and the contents of peer_key is shed along with it. We may have to revert parts of a4c8ff0 to move the field back into SSL3_STATE so we can access the field.
Laster versions of OpenSSL have changed this to an alias to SSL_get_peer_tmp_key which means you can retrieve the client’s key if you’re the server and vice versa (openssl/openssl@a51c9f6). This causes SSL_get_server_tmp_key to have very confusing behavior where you actually retrieve the client's key when you're a server, but Ruby already has tests that's somewhat dependent on the SSL_get_peer_tmp_key behavior.

Testing:

Test that tries retrieving a X25519 or EC_KEY based on the connection for each TLS version.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.05085% with 10 lines in your changes missing coverage. Please review.

Project coverage is 78.77%. Comparing base (48a4057) to head (efb7969).

Files with missing lines	Patch %	Lines
ssl/ssl_lib.cc	67.85%	9 Missing ⚠️
ssl/ssl_key_share.cc	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1945      +/-   ##
==========================================
+ Coverage   78.73%   78.77%   +0.04%     
==========================================
  Files         590      590              
  Lines      101428   101476      +48     
  Branches    14384    14395      +11     
==========================================
+ Hits        79856    79935      +79     
+ Misses      20935    20906      -29     
+ Partials      637      635       -2     

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