SR-2692: Support enabling workload identity during hub registration

[gtrog]

In order to be able to install Anthos Service Mesh 1.9 or higher, it now requires an identity provider to be set:
https://github.com/GoogleCloudPlatform/anthos-service-mesh-packages/blob/1.9.8-asm.1+config1/scripts/asm-installer/install_asm#L2157

The solution for that is to pass the --enable-workload-identity flag when registering with the hub (instead of SA key).

Adding a new variable enable_workload_identity to the hub module that will pass this flag when registering.

Tested happy path in our terraform project by pulling in this module through our forked repo.

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[gtrog]

I'll sign the CLA as soon as legal gives me the green light

[comment-bot-dev]

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• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
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[morgante]

This might be superseded by #947.

[gtrog]

This might be superseded by #947.

Ok, I can close this for now and continue using our fork of the hub module until this is ready.