Add codelist metadata to API response

[iaindillingham]

Add codelist metadata to API response

This adds description, methodology, and references properties (codelist metadata) to a response, following a request with the appropriate query parameters. The following example takes roughly 4s to execute locally.

http://localhost:7000/api/v1/codelist/?description&methodology&references

Closes #1991

Acceptance criteria:

• [x] Ability to retrieve codelist information, including metadata such as description and references, via an API endpoint.
• [x] OSI is still able to retrieve codelist information as expected. (Tested locally.)
• [ ] Let Seb know this work has been done, so that he can inform the developer who wants to have this information.

Closes #1991

[iaindillingham]

Thank you for your comment, @rebkwok. I've added methodology to the response, and controlled each with a separate query parameter. How does the PR look to you, now?

[rebkwok]

LGTM :)