config: add support for extra TLS configuration

[mattsains]

Added support for configuring:

• ClientCertificate
• ClientKey

Based on #6376
Partially addresses #6351

[codecov]

Codecov Report

Attention: Patch coverage is 93.10345% with 4 lines in your changes missing coverage. Please review.

Project coverage is 67.1%. Comparing base (e97e6e4) to head (9e39964).
Report is 35 commits behind head on main.

Files with missing lines	Patch %	Lines
config/config.go	81.8%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6378     +/-   ##
=======================================
+ Coverage   67.0%   67.1%   +0.1%     
=======================================
  Files        193     193             
  Lines      15660   15718     +58     
=======================================
+ Hits       10494   10554     +60     
+ Misses      4876    4875      -1     
+ Partials     290     289      -1     

Files with missing lines	Coverage Δ
config/log.go	95.3% <100.0%> (+0.4%)	⬆️
config/metric.go	85.8% <100.0%> (+0.4%)	⬆️
config/trace.go	97.3% <100.0%> (+0.2%)	⬆️
config/config.go	85.0% <81.8%> (-1.1%)	⬇️

... and 1 file with indirect coverage changes