Improve authentication options for GoogleCloudTranslator

[ababic]

This PR adds the CREDENTIALS_PATH and CREDENTIALS options to GoogleCloudTranslator, giving developers more control and (more importantly) a way to provide service account credentials when storing actual key files isn't an option.

[codecov-commenter]

Codecov Report

Base: 91.78% // Head: 91.66% // Decreases project coverage by -0.12% ⚠️

Coverage data is based on head (8d4e1ce) compared to base (fb4db60).
Patch coverage: 91.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #645      +/-   ##
==========================================
- Coverage   91.78%   91.66%   -0.13%     
==========================================
  Files          46       47       +1     
  Lines        3980     4006      +26     
  Branches      603      608       +5     
==========================================
+ Hits         3653     3672      +19     
- Misses        194      200       +6     
- Partials      133      134       +1     

Impacted Files	Coverage Δ
wagtail_localize/machine_translators/google.py	78.26% <91.66%> (ø)
wagtail_localize/side_panels.py	47.61% <0.00%> (-2.39%)	⬇️

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