Add mod_ssl-like Placeholder Support for Server and Client Certificates in Caddy

[Ko496-glitch]

This pull request introduces functionality similar to Apache's mod_ssl by adding support for dynamic placeholders for server and client certificates in Caddy. The implementation addresses the following:

1)Dynamic Placeholder Population:

Populates placeholders for both server and client certificates during TLS connections, enabling detailed certificate information to be accessed in configurations (e.g., headers, logs).

2)Support for Default Values:

Ensures placeholders return meaningful default values (e.g., null or descriptive strings) when client certificates are not provided (e.g., in non-mTLS scenarios).

3)Centralized Logic:

Introduced a new function, handleMTLSEnabledWithExport, to handle placeholder population efficiently, reducing redundancy and improving maintainability.

[CLAassistant]

All committers have signed the CLA.

[Ko496-glitch]

Updated the lowercase placeholder and comments on the change.

[Ko496-glitch]

@mohammed90 just checking in too see if everything for the PR looks good ?

[mohammed90]

@mohammed90 just checking in too see if everything for the PR looks good ?

At first glance, it looks fine, but I haven't had the time to review it thoroughly. We'll get to it.

[Ko496-glitch]

#6713

[francislavoie]

• This is in conflict with caddyhttp: Evaluate tls.client placeholders more accurately (fix #7530) #7534 (merged) and * feat: add support for base64 encoded client certificate chain #6920 (WIP)
• The code is using spaces rather than tabs, which is incorrect, and messes up the diff. Idiomatic Go uses tabs.
• Setting the values to strings like "No Server Certificate Subject" is strange. Are you doing this to mimic Apache or something? I don't think that's good. I think if anything it should be set to empty (like caddyhttp: Evaluate tls.client placeholders more accurately (fix #7530) #7534 is now doing)
• This needs tests to cover the new functionality, and to ensure no regressions exist.

Thanks for the effort; I think I'll close this for now because it's been a year and would need to be significantly revised anyway. Feel free to make a new PR if you'd like to address it again.