feat: add fields parameter to set_iam_policy for consistency with update methods

[tswast]

Also:

• Add docstrings for get_iam_policy and set_iam_policy
• Add code sample for get_iam_policy and set_iam_policy

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Fixes #1871 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/create_iam_policy_test.py:19, tag bigquery_create_iam_policy

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[tswast]

For inclusion at https://cloud.google.com/bigquery/docs/control-access-to-resources-iam#grant_access_to_a_table_or_view

[tswast]

This test is now redundant with a code sample which serves the same purpose with regards to testing.

[chalmerlowe]

LGTM