[Extensions] Add support for extension_dns list in extensions.yml file

[cwperks]

Similar to the setting plugins.security.nodes_dn which lets a cluster admin define the list of distinguished names that a node of the cluster can use to join the cluster with, extensions need an analogous mechanism. The distinguished name is extracted from the node's transport certificate and identifies a request as coming from a node (though not uniquely - nodes can connect with the same distinguished name and this is even done in the demo configuration!).

This new setting will initially be static, though this needs to also be dynamic in future iterations similar to the nodes_dn.yml dynamic configuration file.

This setting can reside in the extensions/extensions.yml file like so:

extensions:
  - name: hello-world
    uniqueId: hw
    hostAddress: '127.0.0.1'
    port: '4532'
    version: '1.0'
    opensearchVersion: '3.0.0'
    minimumCompatibleVersion: '3.0.0'
    extension_dns:
      - 'CN=extension-01,OU=SECURITY,O=OPENSEARCH,L=BROOKLYN,ST=NEW YORK,C=US'

[cwperks]

This PR introduced a mechanism for an extension to perform transport requests with the cluster and the request is identified through the threadcontext header extension_unique_id which is checked against the registry of installed extensions. In addition to that check it would be strong to also have a check against the distinguished name of the peer certificate used in the transport request.

[peternied]

I would recommend to avoid using acronyms in config files so they are easy to search / gain context on such as distinguished name. DNS also means domain name service which can be miss leading for those unfamiliar.

[cwperks]

@peternied How about support for both extension_dns and extension_distinguished_names? Error out if both are set as an invalid config?

[peternied]

That proposal is overly complicated, I'm just providing feedback about the name

[samuelcostae]

Just commenting here for awareness, im adding this setting at the same time as adding bwcPluginMode setting from issue #2616

[MaciejMierzwa]

Merged: #3031

[davidlago]

@MaciejMierzwa does that mean this issue is ready to be closed? or are there other PRs needed before we do so?