Adds server TLS policy property to target HTTPS proxy resource

[modular-magician]

Fixes #14980

Adds the server_tls_policy property to the TargetHTTPSProxy resource so that it may be configured with mTLS.

I was unsure whether this field should be in the beta or ga provider. The field is available in the v1 API, but the networksecurity.ServerTlsPolicy resource is only available with the beta API. Any guidance here would be appreciated.

This is the last change required for end-to-end deployment of mTLS load balancers with terraform.

Release Note Template for Downstream PRs (will be copied)

compute: Added server_tls_policy argument to target_https_proxy resource

Derived from GoogleCloudPlatform/magic-modules#8857

[github-actions]

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