[l10n/automation] Add comment renderer and manifest validator

[Chaitanya-Keyal]

Note

One of the l10n source-string automation PRs (#940, #941, #942, #943, #944). The manifest schema this validates against, and the tests' conftest, are added in #940.

Description

Problem or Issue being addressed

The review manifest is produced by untrusted PR code but rendered into a PR comment by a trusted job. We need to
(a) validate the manifest before trusting it and
(b) render it without letting attacker-controlled source strings break out of the comment.

Solution

Adds two tools:

• validate_manifest.py -- validates a manifest against the schema and checks two trust anchors GitHub sets authoritatively (the repository, and the head SHA the triggering run was built from). This is the security boundary between the untrusted producer and the trusted consumer.
• render_comment.py -- renders the advisory comment. Source strings are shown inside a ```diff``` fence with newlines flattened and a +/- prefix on every line, so no msgid can start a line and break the fence or inject markup. A test feeds a hostile msgid and proves no breakout.

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This pull request is categorized as a:

• Other (l10n automation)

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Checklist

I ran pytest locally

• All tests passed before submitting the PR

(python -m pytest l10n/automation/tests -- these tests live outside the project's default testpaths and are run explicitly.)

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I included screenshots of any new or modified screens

• N/A

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I added or updated tests

• Yes

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I tested this PR hands-on on the following platform(s):

• Github Actions

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I have reviewed these notes:

• Keep your changes limited in scope.
• If you uncover other issues or improvements along the way, ideally submit those as a separate PR.
• The more complicated the PR, the harder it is to review, test, and merge.
• We appreciate your efforts, but we're a small team of volunteers so PR review can be a very slow process.
• Please only "@" mention a contributor if their input is truly needed to enable further progress.

• I understand

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[wolgwang1729]

LGTM!