[Fleet][POC] Add multiple ES outputs

[mostlyjason]

This issue is for POC work only

In Fleet global settings, users should be able to add multiple Elasticsearch outputs with different settings and names for each. The user can can also add remote clusters by entering a service token for the remote cluster.

• In the Fleet Kibana UI, users can set multiple ES outputs to the same ES cluster used by the Fleet Kibana UI
  • Users can provide a different set of hosts, such as for proxy URLs
  • Users can provide different settings, such as backoff times
  • Work with the new Fleet Server capability to support mulitple outputs [Elastic Agent] Infrastructure to support multiple outputs in a policy beats#27442
• The user can give separate global defaults for integration data and agent monitoring
• Override the default global outputs within an agent policy

Related:

• See more details in the project google doc
• Support customization of throughput relevant settings in APM integration beats#26638
• https://github.com/elastic/cloud/issues/84282

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)