[keyserver] Introduce OLMUtility type

Summary:
The getOLMUtility() function we introduce is going to need to return Promise<OLMUtility>.

Based on the TypeScript typedefs:

Test Plan: Will be tested implicitly by subsequent diffs.

Reviewers: ashoat, tomek, marcin, varun

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6900