Invalid index_id with ingest v2 returns 429

[rdettai]

Description of the issue

When using index templates, specifying and index name that matches the pattern but has illegal characters results in a 429 response code instead of a 400.

Description of the problem:

• the index_id validity is not validated until the metastore is called
• calling GetOrCreateOpenShards with an invalid index_id on the metastore fails with MetastoreError::JsonDeserializeError
• a failure to call the metastore is logged but not repported to the ingest workbench
• because the router is not populated, the workbench fails with "shard not found errors" for the failing index but also other targetted indexes batched with it

Proposed solution

The index id is now validated in quickwit-serve, before calling the ingest router (ES bulk and native APIs)

How was this PR tested?

Added unit and integration (python) tests