[ML] Add request origin to the EIS X-elastic-product-use-case header

[davidkyle]

The X-elastic-product-use-case is change to be a multi-value header and the request origin (search, ingest, unspecified) added to the header for sparse and dense embedding requests. Clients can also set X-elastic-product-use-case which is why it must be a multi-value header.

The 3 potential values are

X-Elastic-Product-Use-Case: internal_search
X-Elastic-Product-Use-Case: internal_ingest
X-Elastic-Product-Use-Case: unspecified

unspecified requests come directly from the REST API

Non issue as it relates to internal comms between Elasticsearch and the Elastic Inference Service

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)