Add field validation for ilm_policy field in package spec.

[kevinlog]

We now have a field to define a custom ilm_policy for a data stream in a package definition. Since the ilm_policy is shipped with the package itself, we should introduce field validation.

See this PR for details on the ilm_policy field: #90

[jsoriano]

Hey @kevinlog,

I am taking a look to this issue, but I am not sure about what validations we can do.

For example I see that the findings data stream of the Cloud Security Posture package indicates as ILM policy logs-cloud_security_posture.findings-default_policy. It also defines an ILM policy in a file called default_policy.json.

Should we validate that the ILM policy matches with something like the following pattern?

{data stream type}-{package name}.{data stream name}-{ilm file name without extension}

[kevinlog]

@jsoriano it has been a while since I opened this ticket, so I am refreshing my memory.

I believe the original reason for opening was that we added support for a custom ilm policy, but there wasn't any validation for the naming convention, some context here.

The convention above looks good to me and would also work with the ilm_policy we defined here: https://github.com/elastic/endpoint-package/tree/master/package/endpoint/data_stream/collection

Our data_stream name: collection
ilm_policy: logs-endpoint.collection-diagnostic