add tests and docs for using custom service account in dataflow flex template job

[modular-magician]

fixes #7658

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

Release Note Template for Downstream PRs (will be copied)

dataflow: added documentation about using `parameters` for custom service account and other pipeline options to `google_dataflow_flex_template_job`

Derived from GoogleCloudPlatform/magic-modules#4260

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