feat: Add Minecraft Account linking endpoints.

[MerchantCalico]

Resolves #3

Adds Minecraft account linking related endpoints for the Discord bot to access.

References:
https://learn.microsoft.com/en-us/entra/identity-platform/v2-oauth2-auth-code-flow
https://minecraft.wiki/w/Microsoft_authentication
https://minecraft.wiki/w/Mojang_API

Little Notes

• The code challenge endpoint is required for the bot's link sent to the end user, it's required because of the frontend being different to the backend.
• The Mojang Public Key is publicly in the repo, which I don't think is really an issue as it's on the Minecraft Wiki page for Microsoft Authentication anyways.
• I have not tested with users who own Minecraft: Java Edition through Xbox Game Pass, I'm unsure if the game ownership code would count for them.

[Oliver-makes-code]

LGTM

[sylv256]

A few things