Made project CAI conversion prefer project number if available

[melinath]

Related to GoogleCloudPlatform/terraform-validator#182. Required for GoogleCloudPlatform/terraform-validator#197 to work properly. See also GoogleCloudPlatform/config-validator#161.

Basically the problem here is that CAI export for projects uses project number instead of project ID (the export format is what config-validator expects to receive.) However, project number is only available after the project has been created. For terraform-validator, we still need a way to handle projects prior to creation (and in particular to disambiguate between them) so we want to use project number if available, but still fall back to project id if necessary.

Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

TF Conversion: Diff ( 1 file changed, 26 insertions(+), 5 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=177916"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccComposerEnvironment_withEncryptionConfig|TestAccNotebooksInstance_notebookInstanceFullExample You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=177932"