Source fallback for Scripting Fields API

[stu-elastic]

If a document is missing a value, the fields API should fall back to source.

If the field is mapped, we should convert the field to the mapped value
Question: Does this work with date formats?

Question: What type should we return if the field is unmapped? A java type? A Json type (see: #53536)?

Users should be able to request source directly and request doc values directly.
Question: Should we indicate the provenance of the value (doc values, source) via an API?

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[jdconrad]

Update:

(#84737) was merged to allow us to go field by field and separate out suppliers for each of the fields. Once that task is complete, we can start adding mapped source fallback, followed up by unmapped source fallback.