allow updating of doc mapper

[trinity-1686a]

Description

this pr aims at laying the foundation work for updating the doc mapping of an index

• [x] store a doc mapper version in the doc mapper
• [x] store a doc mapper version in splits meta
• [ ] allow updating the doc mapper, auto-incrementing its version (metastore level)
• [ ] allow updating the doc mapper (api level)
• [x] block merges between multiple version of a doc mapper
• [ ] restart ingestion pipeline so they notice the change in doc mapper decently quickly (stretch goal, maybe put in a 2nd pull request)

How was this PR tested?

• [x] test we properly block merges (unit tests)
• [ ] test we properly block merges (integration tests)
• [x] test modifying different properties (tokenizer, what is stored...), and how it breaks (or not) search