Stop provider sending empty description fields in IAM conditions, to avoid permadiffs in plans

[modular-magician]

Description

Partially fixes hashicorp/terraform-provider-google#8701

This PR changes the expandIamCondition function, which is used in all IAM _binding and _member resources to read parts of the resources state and convert it into a cloudresourcemanager.Expr variable.

By removing "Description" from the ForceSendFields it stops those variables containing empty Description fields. In the case of the google_iam_policy data source, this means that all the zero valued description fields are ignored when the policy_data JSON is generated from the config input.

This prevents the policy_data JSON containing empty description fields:

"policy_data": "{\"bindings\":[{\"condition\":{👉 \"description\":\"\"👈 ,\"expression\":\"request.host == 'foobar-b.example.com'\",\"title\":\"foobar-b\"},\"members\":[\"group:<email-here>\"],\"role\":\"roles/iap.httpsResourceAccessor\"}]}"

versus

"policy_data": "{\"bindings\":[{\"condition\":{\"expression\":\"request.host == 'foobar-b.example.com'\",\"title\":\"foobar-b\"},\"members\":[\"group:<email-here>\"],\"role\":\"roles/iap.httpsResourceAccessor\"}]}"

If the JSON contains empty description fields there is a perma-diff that is only present in plans after a user makes a legitimate change to the config. Then, the plan contains lots of additional meaningless diffs to add empty descriptions:

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests). N/A
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

iam: fixed permadiff resulting from empty fields being sent in requests to set conditional IAM policies

Derived from GoogleCloudPlatform/magic-modules#6473