Migrate existing indices to a data stream

[danhermann]

Introduce a new API to migrate existing indices into a data stream. The first iteration of the API will operate on a single alias with the following provisions:

• The name of the alias will be used as the name of the new data stream. As part of the migration, the alias will be removed.
• The alias should be "clean" with no filter or routing specifications.
• All indices referenced by the alias will be set to hidden.
• The alias must have a write index.
• The _data_stream_timestamp field mapper will be inserted into the indices to enable validation.
• The mappings of the indices with _data_stream_timestamp meta field mapper will be validated.
• A composable index template must exist for the new data stream. If it is missing, the user must create it first.

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Data streams)

[martijnvg]

@danhermann I think this can be closed now?

[danhermann]

Completed by #61525, #63749, #64415, and #65017.