feat(core): Add new auth class using flare identity server for Amazon Q (#6958)

## Problem
The business logic of auth-related features is complex and implemented
separately for all IDEs. Flare/DEXP LSP server has implemented auth that
can be reused for all IDEs.

## Solution
This PR is the first in a set of PRs to migrate all auth for vscode to
Flare, leveraging their identity server

## Components
* New `auth2` file with all logic to connect to Flare for SSO-related
auth.

## Testing
This PR includes unit tests. The current full implementation is tested
manually and can be found here:
https://github.com/opieter-aws/aws-toolkit-vscode/tree/wip/amazonqLSP-auth

When the auth code is complete, we will conduct a bug bash before
release.

NOTE: integration tests are failing, since this PR only contains the
source code change of `AuthUtil.ts`, and not yet all reference changes.
This is to keep the PR manageable. Follow up PR will contain reference
changes (~90 files).

---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).
- License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: opieter-aws