Add support for OAuth2 applications

[Francessco121]

Users should be able to use DiscordHttpClient on behalf of a user via OAuth2. This also means that each entity needs to be audited to ensure all properties are supported.

• Allow authentication through OAuth2
• Verify OAuth2 only properties for DiscordUser are present
• Support group DMs (Support group DMs #77)
• GET /oauth2/applications/@me (see OAuth 2 topic)
• GET /oauth2/@me