[Feature/extensions] Merge to main tracking - Extension Points, initial REST implementation and registering Transport Actions

[ryanbogan]

This is a tracking issue for the second merge of feature/extensions into main.

PR/Commit to Cherry-Pick:

• Commit ID: 2a49d5a
• [Feature/extensions] Handle named wildcards (REST path parameters) #4415

This merge contains the following PR's in addition to the one above:

• [Feature/extensions] Restructured testExtensionsInitialize #4029
• [Feature/extensions] Extensibility support for getNamedWriteables extension point #3925
• issue #28 Rename to extensions #4063
• [Feature/extensions] Provide Extension API to OpenSearch #4100
• [Feature/extensions] Only send one extension info when initializing #4302
• [Feature/extensions] Register REST API and forward REST requests to associated extension #4282
• [Feature/extensions] Adding support for registering Transport Actions for extensions #4326
• [Feature/extensions] Pass full RestResponse to user from Extension #4356