Use auth tokens passed from core and introduce extension and user REST clients

[cwperks]

Description

Companion example PR for integ testing in security: cwperks/security#6
Companion example PR for integ testing in core: cwperks/OpenSearch#94

This PR introduces an extensionRestClient and userRestClient that an extension developer can utilize to make REST Requests with Auth Tokens back to an OpenSearch Cluster.

• The extensionRestClient is part of the extensionsRunner and is available extension-wide. This REST Client uses a Service Account Token to make requests to an OpenSearch cluster on an extension's own behalf. A common example of an extension making requests to OpenSearch on its own behalf is when interacting with a system index that the extension has reserved.

• The userRestClient is instantiated in handleRequest of BaseExtensionRestHandler and is available to use in all extension REST Handlers. This REST client is utilized to make requests back to opensearch on behalf of the original authenticated user utilizing an On-Behalf-Of token. The On-Behalf-Of token is a short lived access token (JWT) that includes an audience claim that is populated with the destination extension's unique ID. The On-Behalf-Of token lets an extension act as the original user. @peternied is also working on mechanisms for cluster admins to impose further restrictions on these tokens. Details can be found here

The last thing this PR introduces is another security setting to be placed in the extension's settings file: ssl.http.enabled

This setting is used to determine the scheme (http or https) of the REST Client. This is a boolean setting.

Issues Resolved

#887

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[peternied]

Took a quick look - thanks for getting this setup, seems decent to me. Maybe there are a couple of things that could be cleaned up by influencing https://github.com/opensearch-project/OpenSearch/pull/8526/files

[peternied]

Seems like this should be called an onBehalfOfToken both here and on the request object?

What do you think about making the type Optional<> ?

[cwperks]

Agreed. I think this needs to be updated in core. I can create a PR for that.