[ML][Inference endpoints creation] Add support for headers in the OpenAI integration (#241887)

## Summary

This PR is the first part of an effort to add custom header support for
OpenAI in the inference endpoints creation UI.

Because adding new properties to a schema require a 2 step deployment
(see
#228371 (comment)),
2 PRs are needed for this.

step one - the schema changes in this PR
step two - things that can set the config property (so here, the UI) -
PR link to be added


### Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [ ] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)
- [ ] Review the [backport
guidelines](https://docs.google.com/document/d/1VyN5k91e5OVumlc0Gb9RPa3h1ewuPE705nRtioPiTvY/edit?usp=sharing)
and apply applicable `backport:*` labels.

Author: alvarezmelissa87

x-pack/platform/plugins/shared/inference_endpoint/server/routes/index.ts

@@ -24,6 +24,7 @@ const inferenceEndpointSchema = schema.object({
     provider: schema.string(),
     taskType: schema.string(),
     providerConfig: schema.any(),
+    headers: schema.maybe(schema.recordOf(schema.string(), schema.string())),
   }),
   secrets: schema.object({
     providerSecrets: schema.any(),