[Automatic Import] Use Data stream name for data_stream.dataset value in input manifests

[ebeahan]

Describe the feature:

Use the user-supplied data stream name as the data_stream.dataset value instead of a generic placeholder.

Describe a specific use case for the feature:

Custom Fleet integrations default to a generic dataset (data_stream.dataset) value - something like http_endpoint.generic. Since users configure their own options for a custom integration, this workflow makes sense.

Elastic integrations, on the other hand, default to a predetermined naming convention based on the integration package + datastream name for dataset - something like github.issues for the issues data stream from the Github package.

[elasticmachine]

Pinging @elastic/security-scalability (Team:Security-Scalability)