Add a header to be able to send logs or traces to a specified index throught the OTEL gRPC APIs

[fmassot]

Currently, when a user sends logs or traces into Quickwit, data will always be indexed in otel-logs-v0_6 or otel-traces-v0_6.
This is a nice default behavior but I see 3 use cases where it makes sense to let the user defines the index:

• Multi-tenancy use case: a user has a multi-tenant configuration and wants to separate data by tenant.
• Custom index config: a user might want to set an index config different from the default one. He could delete the default index and recreate a new one, but it would be clearer to define its own index.
• Migration use case: we will probably make some updates on the schema and at one point, the user will need to migrate to a new index but will still want to access the old index.

A simple solution to handle those 3 use cases is to pass a custom header to read/write gRPC requests that defines the index ID.

It will also work with Jaeger UI as it's possible to specify a custom header that is added to gRPC requests.

[fmassot]

Fixed by #4089