Allow to configure the IDP trusted CA from a file

[sergicastro]

This also makes the JWKS fetcher use the same config from OIDC instead of using a separated one.
That also adds to JWKS fetcher the use also a customer CA to verify the certs.

This is an initial PR that will be followed by others adding:

• A periodic reloader of the CAs, pre-loading them instead of loading in each request and updating them if they change
• Support for loading the CAs from k8s secrets

This fixes #39

[codecov]

Codecov Report

Attention: Patch coverage is 84.21053% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 90.62%. Comparing base (cfe4795) to head (372211e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
- Coverage   90.95%   90.62%   -0.33%     
==========================================
  Files          17       18       +1     
  Lines        1238     1259      +21     
==========================================
+ Hits         1126     1141      +15     
- Misses         78       81       +3     
- Partials       34       37       +3     

Files	Coverage Δ
internal/authz/oidc.go	93.48% <33.33%> (-0.52%)	⬇️
internal/oidc/jwks.go	96.07% <83.33%> (-3.93%)	⬇️
internal/tls.go	91.30% <91.30%> (ø)

... and 1 file with indirect coverage changes