[Cases] Add bulk attachments internal route

[cnasikas]

Summary

This PR adds the ability to bulk attach attachments to a case. It creates the following internal route:

POST /internal/cases/{case_id}/attachments/_bulk_create

Testing

To test the new endpoint do the following request and check if you see the attachments in the case.

POST /internal/cases/<case_id>/attachments/_bulk_create

[
    {
        "alertId": [
            "40ed218f68f268172de6e86a0eba05f366e25913bee7faeda0ccbc677206a0c8"
        ],
        "index": [
            ".internal.alerts-security.alerts-default-000001"
        ],
        "rule": {
            "id": null,
            "name": null
        },
        "type": "alert",
        "owner": "securitySolution"
    },
    {
        "type": "user",
        "comment": "a comment",
        "owner": "securitySolution"
    }
]

Closes: #124814

Checklist

Delete any items that are not applicable to this PR.

• Unit or functional tests were updated or added to match the most common scenarios

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[cnasikas]

@elasticmachine merge upstream

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)

[academo]

Something that is not clear in the code for me: What happens if we hit the API with 0 attachments? would it fail on the validation? Can you also add a test for it?

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: ebae0cd
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• Webpack Bundle Analyzer

Test Failures

• [job] [logs] Default CI Group #9 / maps app auto fit map to bounds initial location should automatically fit to bounds on initial map load

Metrics [docs]

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
cases	90.6KB	90.7KB	+11.0B

History

• 💛 Build #37267 was flaky d5e2399
• 💚 Build #35782 succeeded f0b0f44
• 💔 Build #35601 failed c0ddc3a
• 💔 Build #35271 failed bc9bf1de9f600b020f561c5450214388cfd7777c
• 💔 Build #35269 failed 7439a554e85a0e111b2c19f78e430175ffaf2ec8

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @cnasikas

[academo]

@cnasikas this is one of those cases I mentioned where things can fail but still succeed. e.g. the attachments are created but the user actions are not.

no need to take action, just a good example of it.

[academo]

Thanks for the test changes!