[DO NOT MERGE] Composer enable_ip_masq_agent flag support (beta) (#9698)

[gor-obr]

Add enable_ip_masq_agent to google_composer_environment (beta)

fixes hashicorp/terraform-provider-google#9698

Do not merge until Composer experiment is fully ramped up.

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• 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).
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• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

composer: added field `enable_ip_masq_agent` to resource `google_composer_environment` (beta)

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I have detected that you are a community contributor, so your PR will be assigned to someone with a commit-bit on this repo for initial review.

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@slevenick, please review this PR or find an appropriate assignee.

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 1 file changed, 7 insertions(+))
Terraform Beta: Diff ( 3 files changed, 20 insertions(+))