feat: break up validate_tls_keys and add unit tests

I've split the existing code in several new functions:
 - load_certificates_from_pem (takes pem data as bytes)
 - build_certificate_chain (takes a list of Certificates)
 - validate_certificate_chain (takes a list of Certificates)
 - validate_key (takes pem data as a string)
 - validate_key_matches_cert (now takes a pem key string and a Certificate)
 - extract_sans (now takes a Certificate)
 - validate_end_entity_expired (now takes a Certificate)
 - validate_end_entity_not_yet_valid (now takes a Certificate)

When a relevant exception arises, these functions raise a type of TLSValidationError,
these are appended to the list of errors when validating a cert.

tests/data/untrusted-root-ca/fullchain.pem

@@ -0,0 +1,57 @@
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+-----BEGIN CERTIFICATE-----
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+-----BEGIN CERTIFICATE-----
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