[FEATURE] Redirect REST API request to appropriate extensions

[dbwiddis]

Is your feature request related to a problem?

See steps 5, 8, and 9 in #64.

What solution would you like?

Following registration, incoming API requests associated with extensions will go to the ExtensionsOrchestrator, where they will be sent to the appropriate extension to handle. The extension will take the appropriate action for that API request, which may include one or more API requests to OpenSearch.

What alternatives have you considered?

Data-heavy API requests likely do not need to be forwarded in their entirety. To minimize latency, it may be useful to separate an API request from its data in some situations.

Do you have any additional context?

See also the DESIGN.md file in #60