Parse user type hints directly to ValuesSourceTypes

[not-napoleon]

In the current implementation, user type hints (specified via the value_type field on aggregations) get parsed leniently through the ValueType enum. The ValueType enum doesn't really add anything however, and we immediately turn that input into a ValuesSourceType. Ideally, we should just parse the user's input directly into a ValuesSourceType

While just getting parity with the current parsing would be sufficient, ideally we would tie into the ValuesSourceRegistry to allow for users hinting VSTs from plugins.

Relates to #42949

[elasticmachine]

Pinging @elastic/es-analytics-geo (:Analytics/Aggregations)

[RobertoMiyoshi]

@not-napoleon May I try to solve this issue? #65916

[not-napoleon]

@RobertoMiyoshi see my comments on the PR. There's a surprising amount of complexity in this issue due to our backwards compatibility policies.

[RobertoMiyoshi]

Hi Mark, thank you for your review and feedback. In this new approach, I worried about the backwards compatibility policies. The previous mapping of user type hints to the ValuesSourceType was based on a Surjective function, i.e., several of ValueTypes can be mapped to the same ValuesSourceType. In order to maintain compatibility with previous version, the PR #66662 is proposing a new mapping through a Bijection, covering the parsing and output the same json that is currently accepted, and also serializing and deserializing the same bytes.