[gcp] Clarify service account privileges

[andrewkroh]

What does this PR do?

Document the specific GCP privileges required by the Pub/Sub input.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Screenshots

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-04-27T02:59:28.538+0000

• Duration: 16 min 27 sec

Test stats 🧪

Test	Results
Failed	0
Passed	28
Skipped	0
Total	28

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (4/4)	💚
Files	100.0% (4/4)	💚 2.857
Classes	100.0% (4/4)	💚 2.857
Methods	100.0% (54/54)	💚 11.913
Lines	96.174% (1106/1150)	👍 7.34
Conditionals	100.0% (0/0)	💚