[ML][AI Connector] Add support for headers in the OpenAI integration: update schema (elastic#239002)

## Summary

This PR is the first part of an effort to add custom header support for
OpenAI in the AI Connector.
Because adding new properties to connector schema require a 2 step
deployment (see
elastic#228371 (comment)),
2 PRs are needed for this.

- step one - the schema changes, and anything else that does NOT set the
config property - done in this PR
- step two - things that can set the config property (so here, the UI)
done here elastic#238710


### 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.

---------

Co-authored-by: Elastic Machine <[email protected]>
Co-authored-by: kibanamachine <[email protected]>

Author: 3 people

x-pack/platform/plugins/shared/actions/server/integration_tests/__snapshots__/connector_types.test.ts.snap

@@ -19,6 +19,7 @@ export const ConfigSchema = schema.object({
   providerConfig: schema.object({}, { unknowns: 'allow', defaultValue: {} }),
   taskTypeConfig: schema.object({}, { unknowns: 'allow', defaultValue: {} }),
   contextWindowLength: schema.maybe(schema.number({})),
+  headers: schema.maybe(schema.recordOf(schema.string(), schema.string())),
 });
 
 export const SecretsSchema = schema.object({

x-pack/platform/plugins/shared/stack_connectors/common/inference/schema.ts

@@ -119,7 +119,7 @@ export class InferenceConnector extends SubActionConnector<Config, Secrets> {
   }
 
   /**
-   * responsible for making a esClient inference method to perform chat completetion task endpoint and returning the service response data
+   * responsible for making a esClient inference method to perform chat completion task endpoint and returning the service response data
    * @param input the text on which you want to perform the inference task.
    * @signal abort signal
    */
@@ -182,7 +182,7 @@ export class InferenceConnector extends SubActionConnector<Config, Secrets> {
   }
 
   /**
-   * responsible for making a esClient inference method to perform chat completetion task endpoint and returning the service response data
+   * responsible for making a esClient inference method to perform chat completion task endpoint and returning the service response data
    * @param input the text on which you want to perform the inference task.
    * @signal abort signal
    */