MTLS Client cert and key as separate fields

What does this MR do?

mTLS client-cert and client-key as separate fields instead of client-cert-key-pairs

More context: !978 (comment 1910557896)

Related issue: Feature Request - Support mutual TLS when calli... (#548)

Changelog: other

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines