Add `index_auto_create` flag per index request

[benwtrent]

The following scenario has caused a number of bugs and many hours of frustrations:

• We desire an alias to be created with a backing concrete index(es)
• There is one place in the code that forgets to create the alias + concrete indices ahead of time (either through programmer error, or a race condition)
• The alias is created as a concrete index and then must be correct through manual means (user intervention on system indices).

To prevent this type of problem, it would be better to fail fast and allow the code paths to recover and optionally create the alias.

A flag on (bulk) index requests that disallow the index to be auto created would be LOVELY. This would help prevent silent failures and enable error handling paths fix the fast failures.

//CC @dakrone

[elasticmachine]

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

[benwtrent]

//CC @elastic/ml-core