feat(chart): add a default service account for runner pods

[LucasMrqes]

The default service account used for runner pods is called burrito-runner(see internal/controllers/terraformrun/pod.go).
Currently, you either:

• Create this specific service account in tenants
• Create a service account with an other name, and reference it with overrideRunnerSpec.serviceAccountName in layers and/or repositories

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.99%. Comparing base (d1a7ba6) to head (3fcbe04).

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #416   +/-   ##
=======================================
  Coverage   50.99%   50.99%           
=======================================
  Files          73       73           
  Lines        4334     4334           
=======================================
  Hits         2210     2210           
  Misses       1935     1935           
  Partials      189      189           

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