Add OpenShift AI REST API specification

[Jan-Kazlouski-elastic]

This PR adds the spec file that was added in this PR:

Specification PR elastic/elasticsearch-specification#5662

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[Jan-Kazlouski-elastic]

This PR is to be merged only after:

• Original PR is merged Implement OpenShift AI integration for chat completion, embeddings, and reranking #136624
• Specification PR is approved Add OpenShift AI integration specifications elasticsearch-specification#5662.

If there are changes to Specification PR - this PR also needs to be updated accordingly.

Labels are to be taken from similar PR:
#132724

CC @DonalEvans

[elasticsearchmachine]

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

[DonalEvans]

I think this should be "enum"

[Jan-Kazlouski-elastic]

At the time this PR was opened, task_type was still defined as a string in every other place. It was converted to an enum in other parts of the codebase during the time this PR was awaiting review. I’ve updated this PR accordingly to align with the latest changes. Thanks.

[DonalEvans]

In addition to the "type" and "description" fields, I think there should be an "options" one listing the valid values for "task_type" for this integration.

[Jan-Kazlouski-elastic]

Same thing here. It was added in other parts of the codebase during the time this PR was awaiting review. I’ve updated this PR accordingly to align with the latest changes. Thanks.