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]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ Ko496-glitch
❌ keenu7ohlan
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[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