feat: OAuth authentication to GitLab.com (!567) · Merge requests · GitLab.org / gitlab-vscode-extension

This MR implements OAuth token refreshing and enables the OAuth Authorization flow.

The MR creates the TokenExchangeService which is now responsible for:

Doing the initial OAuth exchange (code for the token) (step 2. of the PKCE flow) (first commit)
Doing the subsequent token refreshes (step 3. of the PKCE flow) (second commit)

I recommend reviewing the MR commit-by-commit.

feat: OAuth authentication to GitLab.com