sources/oauth/discord: Allow changing Discord API URLs

# Feature Request: Configurable Discord API URLs for Auth Source

## Is your feature request related to a problem? Please describe.
Currently, it is not possible to modify the Discord URLs used by the Discord Auth Source. Specifically, the `access_token_url` and `profile_url` endpoints are hardcoded, preventing users from routing requests through a proxy.

Proxies are widely used in larger Discord bots to avoid hitting rate limits. Without configurable URLs, users have to rely on fragile workarounds, which can lead to instability and higher maintenance overhead.

Some widely used Discord API proxies include:

- [Nirn Proxy](https://github.com/germanoeich/nirn-proxy)
- [Twilight HTTP Proxy](https://github.com/twilight-rs/http-proxy)
- [@discordjs/proxy](https://github.com/discordjs/discord.js/tree/main/packages/proxy)

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## Describe the solution you'd like
I would like the Discord Auth Source to allow configuring the following URLs:

- `access_token_url`
- `profile_url`

For example:

- **Access Token URL**: `https://my-proxy.proxy.svc.cluster.local:8080/api/oauth2/token`
- **Profile URL**: `https://my-proxy.proxy.svc.cluster.local:8080/api/users/@me`

Currently, setting `urls_customizable = True` works locally, but it exposes many other fields unrelated to Discord, which can confuse users. Ideally, only `access_token_url` and `profile_url` should be customizable, while `authorization_url` should remain fixed, since users always need to be redirected to Discord for authentication.

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## Describe alternatives you've considered
One alternative is modifying DNS or `/etc/hosts` in the container to point `discord.com` to the proxy. This approach is hacky, error-prone, and hard to maintain across environments.

Properly configurable URLs would provide a clean, maintainable solution.