Refactor async index creation tasks

What does this MR do and why?

Related to #389638

It adds two new rake tasks for handling async index execution, one for each database(the end goal is to use these tasks in production to create the indexes):

bin/rake gitlab:db:execute_async_index_operations:ci
bin/rake gitlab:db:execute_async_index_operations:ci[3] # optional param for how many instruction to execute per invocation, default 2

bin/rake gitlab:db:execute_async_index_operations:main
bin/rake gitlab:db:execute_async_index_operations:main[4]

It also adds new columns to the pending indexes table to skip indexes that fail creation and saves the error for later(> 1 week) debugging.

And one more task for ease of development:

bin/rake gitlab:db:execute_async_index_operations:all

invokes the tasks on each database, with a default pick of 1000. This should be used mostly for development to test the async index creation for migrations and it's using a large pick number because there might be other entries in the queue.

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