Add support for using GCP KMS in example-gcp

[AlCutter]

This PR enables example-gcp to use GCP's KMS support for Ed25519 signatures.

Towards #7

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 78 lines in your changes missing coverage. Please review.

Project coverage is 35.17%. Comparing base (46ec9c2) to head (9c1ac8c).
Report is 69 commits behind head on main.

Files	Patch %	Lines
cmd/example-gcp/kms.go	0.00%	60 Missing ⚠️
cmd/example-gcp/main.go	0.00%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #153      +/-   ##
==========================================
- Coverage   35.80%   35.17%   -0.64%     
==========================================
  Files          16       33      +17     
  Lines        1363     2883    +1520     
==========================================
+ Hits          488     1014     +526     
- Misses        801     1759     +958     
- Partials       74      110      +36     

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[mhutchinson]

I guess you'll be updating the terraform to match. Docs (or ideally automated docs like terraform) for setting up this more complex example would be good.

[AlCutter]

Terraform for this has been added to #144, just trying to make sure it all works more or less as intended before I send that over to you.