feat(client): Add support for SNI override

[AlisonT1230]

Summary

Add ability to explicitly set SNI for HTTPS and TLS endpoint monitoring.

Note:

• The logic around checking if a TLS config exists has been modified in this PR. Previously, it required the presence of the client certificate & key. This PR changes it to check just for existence of client.tls field in order to make room for configuring SNI without needing to provide a client certificate & key.
• Updated the MTLS docker compose example to make use of the sample certs in that folder so there's a setup that is able to do secure mTLS connection. The gatus spun up in the environment is rewrapped such that it expects those test certificates as a valid CA.

related to #907

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Updated documentation in README.md, if applicable.