Create Slack integration for Google Cloud Build

[RJPercival]

Posts notifications to a Slack channel whenever a Google Cloud Build completes. We've been using this in our #dev channel for months now.

Checklist

• [ ] I have updated the CHANGELOG.
  • Adjust the draft version number according to semantic versioning rules.
• [ ] I have updated documentation accordingly (including the feature implementation matrix).

[codecov[bot]]

Codecov Report

Merging #1969 (df4e943) into master (5e12fb3) will decrease coverage by 7.99%. The diff coverage is n/a.

:exclamation: Current head df4e943 differs from pull request most recent head 1e56584. Consider uploading reports for the commit 1e56584 to get more accurate results

@@            Coverage Diff             @@
##           master    #1969      +/-   ##
==========================================
- Coverage   66.12%   58.12%   -8.00%     
==========================================
  Files         103      113      +10     
  Lines        7013     9676    +2663     
==========================================
+ Hits         4637     5624     +987     
- Misses       1884     3555    +1671     
- Partials      492      497       +5     

Impacted Files	Coverage Δ
storage/cloudspanner/log_storage.go	0.00% <0.00%> (-55.93%)	:arrow_down:
merkle/log_proofs.go	44.23% <0.00%> (-52.74%)	:arrow_down:
storage/cloudspanner/admin.go	5.92% <0.00%> (-39.23%)	:arrow_down:
storage/cloudspanner/tree_storage.go	0.00% <0.00%> (-36.32%)	:arrow_down:
storage/mysql/queue.go	43.13% <0.00%> (-19.61%)	:arrow_down:
trees/optype_string.go	16.66% <0.00%> (-16.67%)	:arrow_down:
quota/group_string.go	22.22% <0.00%> (-11.12%)	:arrow_down:
quota/kind_string.go	25.00% <0.00%> (-8.34%)	:arrow_down:
storage/tree/layout.go	67.74% <0.00%> (-7.26%)	:arrow_down:
quota/quota.go	85.71% <0.00%> (-5.96%)	:arrow_down:
... and 123 more

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[roger2hk]

/gcbrun

[JAORMX]

Let's close this in favor of https://github.com/google/trillian/pull/2863