refactor: Set up OAuthClient and oauthCallback entrypoint

• gitlab-web-ide Spike MR
• ...
• Part 1 - refactor: Add AuthType to support OAuth (!247 - merged)
• Part 2 - refactor: Set up OAuthClient and oauthCallback ... (!250 - merged)

Description

• In the example package, this MR sets up the Auth Type dropdown with None, Token, and OAuth (In Development). The OAuth one will not work yet.
• In the new oauth-client package, this MR sets up the basic interface of this package with an OAuthClient class that has stubbed out methods.
• In the vscode-bootstrap package, we add oauth handling to the getAuthProvider
• In the main web-ide package, we introduce an entrypoint for oauthCallback which is expected to be called by gitlab.com/-/ide/oauth_callback. See the spike MR's for more context !240 (closed).

Screenshots

Auth: None and Auth: Token still work as expected 🎉

webide_part2_demo