fix: make safeAppId optional, add messagesTag + message endpoint

[iamacook]

What it solves

1. Marks safeAppId as optional (which matches that of the transaction service and the gateway here). No safeAppId is present when testing adding a custom app.
2. Adds getSafeMessage endpoint for retrieving a SafeMessage by hash. It does not return a type as seen here.
3. Adds messagesTag to Safe info endpoint (as seen here).

[github-actions]

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Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	0	N/A

• Result: ✅ success
• Annotations: 0 total

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^{Report generated by eslint-plus-action}

[katspaugh]

👍