fix: initial config should not influence sslv2

[jmayclin]

Resolved issues:

#4585

Description of changes:

This PR separate out parsing and processing for sslv2 formatted client hellos. Previously both items were handled by s2n_sslv2_client_hello_recv. Splitting these out into two functions allows us to

1. parse the sslv2 client hello without touching anything on the config
2. invoke the client hello callback
3. then process the sslv2 client hello (which requires touching the config)

Call-outs:

This code is basically stolen respectfully appropriated from an earlier draft of #4676 , so thanks @maddeleine!

Testing:

I added a unit test that confirms the config isn't de-referenced in the parsing of the SSLV2 client hello.

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

[jmayclin]

conn->server_protocol_version is set to s2n_highest_protocol_version in s2n_connection_new. s2n_highest_protocol_version is either TLS 1.2 or TLS 1.3, depending on whether s2n_disable/enable_tls13_test functions have been used.

Previously

server_protocol_version was never modified for SSLv2 client hellos. This line was just asserting on s2n_highest_protocol_version which was toggled between test runs by s2n_disable/enable_tls13_test.

Proposed

server_protocol_version is now modified by this section in process_client_hello.

s2n-tls/tls/s2n_client_hello.c

Lines 535 to 538 in cc2ea72

	if (!s2n_connection_supports_tls13(conn) || !s2n_security_policy_supports_tls13(security_policy)) {
	conn->server_protocol_version = MIN(conn->server_protocol_version, S2N_TLS12);
	conn->actual_protocol_version = MIN(conn->server_protocol_version, S2N_TLS12);
	}

From my reading/audit of the codebase, server_protocol_version is mostly used for various checks and assertions related to downgrade protection. From the brief audit that I did, I didn't see any places that this would be a noticeable behavior change, but this part of the codebase is pretty new to me and our SSLV2 test coverage is pretty low ☹️

I'll also throw out that server_protocol_version being set to TLS13 while using SSLV2 client hellos seems suspicious given this RFC requirement

Implementations MUST NOT negotiate TLS 1.3 or later using an SSL version 2.0 compatible CLIENT-HELLO.
https://www.rfc-editor.org/rfc/rfc8446#appendix-D.5

Note: I think the code chunk above should just always be assigning S2N_TLS12. When I made that change, all the test cases continued to pass exception one of the renegotiate ones. Need to investigate further