Prepare OTel Elasticsearch exporter with the credentials of the output - #5469

What is the problem this PR solves?

Configure credentials for Elasticsearch OTel exporters.

How does this PR solve the problem?

It copies the credentials from the referenced output when their types match as Elasticsearch.

How to test this PR locally

Use the kibana code from elastic/kibana#233090. Enable the feature flag enableOtelIntegrations.

Using elastic-package to install the scenario (with these instructions: https://github.com/elastic/elastic-package/blob/main/docs/howto/use_existing_stack.md).

mage build:cover docker:customagentimage
export ELASTIC_AGENT_IMAGE_REF_OVERRIDE=<image tag generated in previous step>
export ELASTIC_PACKAGE_ELASTICSEARCH_PASSWORD=changeme
export ELASTIC_PACKAGE_ELASTICSEARCH_USERNAME=elastic
export ELASTIC_PACKAGE_KIBANA_HOST=http://localhost:5601
export ELASTIC_PACKAGE_ELASTICSEARCH_HOST=http://localhost:9200
elastic-package stack up -v -d --provider environment --version 9.2.0-SNAPSHOT

Then install or test the httpcheck package from https://github.com/elastic/elastic-package/tree/main/test/packages/parallel/httpcheck.

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

• Follow up to Extend schemas to support OTel configuration for Hybrid agents #5241.
• Follow up to Complete OTel configuration pipelines with an exporter kibana#233090.
• Closes https://github.com/elastic/ingest-dev/issues/5712.