support updating doc mapper through api

[trinity-1686a]

Description

allow updating the doc-mapper of an index
part of #5084

How was this PR tested?

updated tests, tested manually on a simple doc-mapper change (changing tokenizer)

[rdettai]

The docs need to be updated:

• quickwit/docs/reference/rest-api.md

  Lines 311 to 333 in 3df3eed

  	### Update an index
  	```
  	PUT api/v1/indexes/<index id>
  	```
  	Updates the configurations of an index. This endpoint follows PUT semantics, which means that all the fields of the current configuration are replaced by the values specified in this request or the associated defaults. In particular, if the field is optional (e.g. `retention_policy`), omitting it will delete the associated configuration. If the new configuration file contains updates that cannot be applied, the request fails, and none of the updates are applied. The API accepts JSON with `content-type: application/json` and YAML with `content-type: application/yaml`.
  	- The retention policy update is automatically picked up by the janitor service on its next state refresh.
  	- The search settings update is automatically picked up by searcher nodes when the next query is executed.
  	- The indexing settings update is not automatically picked up by the indexer nodes, they need to be manually restarted.
  	#### PUT payload
  	| Variable | Type | Description | Default value |
  	|---------------------|--------------------|-----------------------------------------------------------------------------------------------------------------------|---------------------------------------|
  	| `version` | `String` | Config format version, use the same as your Quickwit version. | _required_ |
  	| `index_id` | `String` | Index ID, must be the same index as in the request URI. | _required_ |
  	| `index_uri` | `String` | Defines where the index files are stored. Cannot be updated. | `{current_index_uri}` |
  	| `doc_mapping` | `DocMapping` | Doc mapping object as specified in the [index config docs](../configuration/index-config.md#doc-mapping). Cannot be updated. | _required_ |
  	| `indexing_settings` | `IndexingSettings` | Indexing settings object as specified in the [index config docs](../configuration/index-config.md#indexing-settings). | |
  	| `search_settings` | `SearchSettings` | Search settings object as specified in the [index config docs](../configuration/index-config.md#search-settings). | |
  	| `retention` | `Retention` | Retention policy object as specified in the [index config docs](../configuration/index-config.md#retention-policy). | |

• quickwit/docs/configuration/index-config.md

  Line 17 in 3df3eed

  Configuration is generally set at index creation and cannot be modified, except for some specific attributes like the search settings and retention policy, which can be changed using the [update endpoint](../reference/rest-api.md) or the [CLI](../reference/cli.md).