Allow specifying Spanner emulator host via Config

[TangoEnSkai]

Context

close #73

When wrench is used as a library, the only way to point it at a Cloud Spanner emulator today is the SPANNER_EMULATOR_HOST environment variable. As the issue notes, calling os.Setenv from test code is process-wide and racy, which is exactly the situation where an emulator is most useful. @kazegusuri welcomed a PR for this in the issue thread; the original reporter never followed up, so here it is.

What

• Add an EmulatorHost field to spanner.Config.
• When set, NewClient appends the same three client options that the Cloud Spanner client libraries build from SPANNER_EMULATOR_HOST (see spanner.NewClientWithConfig and the admin client's init.go hook): a passthrough:/// endpoint with the scheme stripped, an insecure gRPC transport, and option.WithoutAuthentication().
• CredentialsFile is ignored when EmulatorHost is set — the emulator takes no credentials, and combining WithCredentialsFile with WithoutAuthentication would fail dial-settings validation.
• Tests:
  • TestEmulatorClientOptions (pure unit test) pins the endpoint normalization for http://, https://, and passthrough:/// prefixes.
  • TestNewClientWithEmulatorHostConfig (emulator integration test) clears SPANNER_EMULATOR_HOST via t.Setenv and verifies that both the data client and the admin client reach the emulator through Config.EmulatorHost alone (CreateDatabase → EnsureMigrationTable → DropDatabase).

Why

The field mirrors the client libraries' own emulator wiring rather than inventing a new path, so behaviour stays identical to SPANNER_EMULATOR_HOST — including the scheme-stripping regex. Existing behaviour is unchanged when EmulatorHost is empty. The new options are appended after Config.ClientOptions, consistent with the documented "config fields override ClientOptions" ordering already used by CredentialsFile.

Completion Criteria

• go build ./..., go vet ./..., gofmt -l . clean
• TestEmulatorClientOptions passes locally (no emulator needed)
• Emulator integration tests pass in CI (TestNewClientWithEmulatorHostConfig runs under make test)

[TangoEnSkai]

@kazegusuri you mentioned in #73 that a PR for this would be welcome — the original reporter never followed up, so here it is. The implementation mirrors the client libraries' own SPANNER_EMULATOR_HOST wiring. PTAL when you have a moment.

[TangoEnSkai]

Following up on the Config option for the Spanner emulator host: the PR is still mergeable, with tests, security scan, and CLA checks passing. @kazegusuri could you review the approach when convenient, or let me know if you would prefer any changes? Thanks!

[sinmetal]

@TangoEnSkai Thank you. I have merged #164 first, so could you please resolve the conflicts?