[LS] Support bearer/token authentication for proxy authorization

Problem

In [LS] Support proxy authentication (#159 - closed) Basic authentication support was introduced for proxy authorization but users may need to use Bearer/token authentication instead.

Considerations

We want to make sure we're properly handling proxy authentication and giving our users tools to adhere to their corporate security policy and provide a way forward to authenticate with our servers. Our focus is on compliance and clear communication about authentication needs.

Proposal

Start with basic proxy detection and manual configuration. We'll need to be able to detect HTTP 407 errors and surface them to the user, as well as provide them with a way to manually configure proxy settings. Update docs to instruct users where to go.

Logs

2024-01-31T11:06:22:811 [info]: Checking token for OAuth validity
2024-01-31T11:06:22:811 [debug]: fetch: request for https://gitlab.com/oauth/token/info made with proxy agent.
2024-01-31T11:06:22:877 [debug]: fetch: request to https://gitlab.com/oauth/token/info returned HTTP 407 after 66 ms
2024-01-31T11:06:22:877 [error]: Error performing token check