Add basic emulator support with updated google-cloud-spanner

[aeroastro]

@jiren

This is not fully-functional support for Spanner Emulator, but the first step.

I have updated the google-cloud-spanner to 2.4 to include googleapis/google-cloud-ruby#8416,
and pass the emulator_host to Google::Cloud::Spanner.new.

This makes the following code working.

ActiveRecord::Base.establish_connection(
  adapter: :spanner,
  project: 'test-project',
  instance: 'test-instance',
  database: 'test-database',
  emulator_host: 'localhost:9010'
)

ActiveRecord::Base.tap(&:connection).connected?
# => true

The reasons why I omit the test codes are:

• This part is not something to be fully tested.
• Actually, I did not find existing test on checking the arguments passed to Google::Cloud::Spanner
• In the future, I would like to run acceptance test with Spanner Emulator on CI, which is appropriate for testing the functionality

Although I confirmed that Spanner Emulator has an issue on ALTER TABLE, this introduced support will helps us a lot.

GoogleCloudPlatform/cloud-spanner-emulator#16

c.f. #48

[skuruppu]

@aeroastro, you can try putting a config similar to that of https://github.com/googleapis/google-cloud-ruby/blob/master/.github/workflows/spanner-integration-tests-against-emulator.yaml to get the acceptance tests running on this repo against the emulator. Let me know if you'd like help with it.

[aeroastro]

@skuruppu
Yes, I am willing to do that.
There are several things I would like you to consider:

• I am going to be busy this March, so implementing the emulator in CI would be scheduled later.
• I confirmed there is a need to change some code in acceptance Rake Task, in addition to skipping table ALTER TABLE. So, I would like to implement it with another Pull Request.

[skuruppu]

@jiren if you could please review this PR, that would be awesome.

[jiren]

@aeroastro Thanks