feat: Add public API endpoint to fetch user sessions

[JerrySentry]

ref: https://github.com/codecov/internal-issues/issues/723

Add a new public V2 API endpoint to return list of users' login session status:

GET https://api.codecov.io/api/v2/{service}/{owner_username}/user-sessions?page={page}&page_size={page_size}

Returns a paginated list of users' login session for the specified owner (org)

Note: Only callable if user is an admin of the org.

Params:

service (string): [required] Git hosting service provider
owner_username (string): [required] username from service provider
page (integer): A page number within the paginated result set.
page_size (integer): Number of results to return per page.

Response:

{
    count: integer,
    next: uri, 
    previous: uri,
    results: [{
        name: string,
        username: string,
        has_active_session: boolean,
        expiry_date: date-time
    }]
}

has_active_session: user has a session and its expiry date is in the future
expiry_date: the longest remaining login session timestamp

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. In 2022 this entity acquired Codecov and as result Sentry is going to need some rights from me in order to utilize my contributions in this PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

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Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.26%. Comparing base (f203361) to head (c4c2a34).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #840   +/-   ##
=======================================
  Coverage   96.25%   96.26%           
=======================================
  Files         814      814           
  Lines       18656    18680   +24     
=======================================
+ Hits        17958    17982   +24     
  Misses        698      698           

Flag	Coverage Δ
unit	92.52% <100.00%> (+0.01%)	⬆️
unit-latest-uploader	92.52% <100.00%> (+0.01%)	⬆️

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