feat: Implement private key store backed by GCP KMS (#1)

https://cloud.google.com/security-key-management

Fixes relaycorp/relayverse#37

TODO

- [x] Session keys: Use CRC32C to verify integrity: https://cloud.google.com/kms/docs/data-integrity-guidelines#calculating_and_verifying_checksums
- [x] Identity keys: Use CRC32C: https://cloud.google.com/kms/docs/data-integrity-guidelines
- [x] Verify the `.name` of the resource in each response: https://cloud.google.com/kms/docs/data-integrity-guidelines#verifying_resource_names
- [x] Wrap API call errors, since their stack traces are utterly useless to infer context.
- [x] Enable timeouts in all API calls, especially Datastore.
- [x] Enable retries in all API calls.
- [x] Remove code to automagically pick unlinked version (first run)
- [x] Write functional tests against actual GCP APIs.

.npmignore

@@ -1,8 +1,9 @@
 *
 
 !build/main/**
+build/main/testUtils
 !build/module/**
-_test_utils.*
+build/modules/testUtils
 *.spec.js*
 
 !LICENSE

docs/gcp.md

@@ -0,0 +1,5 @@
+# GCP-powered Awala key stores
+
+## Limitations
+
+- All the GCP resources must be located in the same GCP project and region.

jest.config.js

@@ -29,7 +29,7 @@ module.exports = {
 
   // An array of regexp pattern strings used to skip coverage collection
   coveragePathIgnorePatterns: [
-    "_test_utils\.[tj]s",
+    "/testUtils",
   ],
 
   // A list of reporter names that Jest uses when writing coverage reports