Make a DAG for pointing ES aliases

[stacimc]

Problem

#2372 adds a DAG for creating ES indices, but does not have any way for promoting the index/applying an alias.

Description

For this issue, we should create a DAG that takes the following conf options:

• source_index: the name of the index that we want to apply an alias to
• target_alias: the name of the alias we want to apply
• delete_old_index: a bool, default to False. Whether to delete the index previously pointed to by the target_alias, if applicable

It should do the following:

• Attempt to get the current index with the given target_alias (it should skip if no such index exists)
• If the target alias is already being used, remove it from the index it currently points to.
• Apply the target_alias to the source_index
• If delete_old_index is True, and we found an index in the first step, delete it
• Report to slack

We should package this logic in a reusable TaskGroup, and also add it as an additional step to the create_new_es_index DAG from #2372.

Alternatives

Alternatively, we could trigger this DAG from the create_new_es_index DAG. If possible it would be better to have those steps actually part of the index creation DAG, but simply reuse the code. This makes it much easier to follow when reading through a DagRun.