Check if embedded cert is base64 encoded before decoding

[jacobtomlinson]

Fixes #484

Instead of trying to base64 decode the certs and failing when they are stored in plain-text this PR checks if they are certs and only decodes them if they are.

This fixes configs which contain new lines in their base64 encoded data (which is technically not valid base64, but decodes fine).

This should not break folks who have plain text certs, which is what the previous try/except behaviour was enabling. Added a test to verify this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.16%. Comparing base (87063fc) to head (01fc4ab).
Report is 111 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #485      +/-   ##
==========================================
+ Coverage   94.61%   95.16%   +0.54%     
==========================================
  Files          29       30       +1     
  Lines        3141     3933     +792     
==========================================
+ Hits         2972     3743     +771     
- Misses        169      190      +21     

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