Enable client reuse when authenticating with app user access tokens

[kyle-leonhard]

This PR allows octocrab instances to be reused with different Github app user access tokens, similar the installation function enabling an instance to be reused with different installations of an app.

The motivation is to avoid creating a new connection every time a request needs made for a different user, user access token. I didn't see any good way to reuse or provide the connection created in build (code)

Here's how it looks:

let octocrab = OctocrabBuilder::new().build()?;

// Later, use the client with many, varying access tokens.
let octocrab = octocrab.user_access_token(token)?

An alternative is to expose an Octocrab::toBuilder method to effectively clone the existing client with varying configuration. This looks hard given how configuration ends up embedded in service layers.

User access token documentation: https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/generating-a-user-access-token-for-a-github-app

[kyle-leonhard]

Immediately after posting this PR I found:

octocrab/examples/custom_client.rs

Line 22 in fae5b08

.with_service(client)

.

I think that'd solve my issue. Though there is still value in enabling reuse in the manner of this PR. I'm hoping maintainers will have an opinion