Composer enable_ip_masq_agent flag support (beta) (#9698)

[modular-magician]

Add enable_ip_masq_agent to google_composer_environment (beta)

fixes hashicorp/terraform-provider-google#9698

I should be applied instead of similar PR: GoogleCloudPlatform/magic-modules#5142

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Release Note Template for Downstream PRs (will be copied)

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

Derived from GoogleCloudPlatform/magic-modules#5277