feat: Access token getter

[kysely]

Description

This PR adds getAccessToken method.

Motivation and Context

We have new clients that use access token when interacting with our backends. They need to manually get and keep track of access token that 1) isn't exposed from the client 2) expires every 15 minutes.

The new getAccessToken method is a simple interface for getting the always up-to-date token.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTION_GUIDE document.
• [x] I haven't repeated the code. (DRY)
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[freaz]

Way better would be to work with baseUrl in fetch. To avoid what Jan mentioned and use existing authentication flow.

What do you need to call? Edgar OneService?

[kysely]

You're right — the frontend needs to call 1) engine API 2) OneService.

With OneService it would be a bit hacky since GraphiQL uses single absolute URL instead of baseURL + path but probably still better than this solution. I'll add that.

[kysely]

Attempt at #90