Support clang 16 in docker CI

[godexsoft]

Fixes #1175

• Adds clang-16 as a secondary compiler to the docker CI image
• Slightly improves how gcc-12 is integrated
• Sets up the CI to make use of clang and run tests compiled by it alongside gcc's build
• These are not the robots you are looking for

Currently clang-tidy is producing a lot of interesting output. But i think this can be handled outside of this PR now.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 64.44%. Comparing base (1fe42c8) to head (1fd2018).

:exclamation: Current head 1fd2018 differs from pull request most recent head d6ce1f9. Consider uploading reports for the commit d6ce1f9 to get more accurate results

Files	Patch %	Lines
src/etl/impl/LedgerLoader.hpp	0.00%	2 Missing :warning:
src/etl/impl/SubscriptionSource.cpp	66.66%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1348 +/- ## =========================================== + Coverage 64.38% 64.44% +0.05% =========================================== Files 248 248 Lines 10544 10544 Branches 5839 5839 =========================================== + Hits 6789 6795 +6 + Misses 1920 1914 -6 Partials 1835 1835 ```

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