Add exists? method to google_compute_address

[ivantsepp]

This adds an exists? method to google_compute_address. The documentation for google_compute_address shows:

describe google_compute_address(project: 'chef-inspec-gcp', location: 'us-west2', name: 'gcp-inspec-test') do
        it { should exist }
end

but that will fail because there is no exists? method. This PR fixes that.

I'm not sure if this is affected by the magic modules I see in the codebase. Also unsure if y'all would prefer if we fix the documentation since there's already an address_ip_exists method.

Let me know if this is the right approach and I can also look into adding tests!

[skpaterson]

Welcome and many thanks for this @ivantsepp ! Also, apologies for the delay in approval.

[ivantsepp]

No worries at all! Thank you for taking the time to look/review this PR!