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/letsencrypt-issued/chain.pem

@@ -0,0 +1,26 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----