Add optional OIDC avatar fetching from the picture claim

[rubentalstra]

This update enables BookStack to optionally fetch user avatars from the OIDC picture claim. The implementation:

1. Introduces a fetch_avatars config flag in config/oidc.php to toggle avatar retrieval.
2. Uses UserAvatars->assignToUserFromUrl($user, $picture, $accessToken) to support both public and private (Bearer token-protected) endpoints.
3. Fetches the picture claim from the user’s ID token or userinfo response, if provided.
4. Works with various OIDC providers (Google, Okta, Keycloak, Azure, etc.), provided they include or allow retrieving a valid picture URL.
5. Includes SSRF protection notes, advising users to only enable this if they trust the domains in the picture field.

This approach does not break existing behavior; avatar fetching is off by default. If enabled, BookStack will try to update a user’s avatar upon login, using the token to authenticate if necessary.

[ssddanbrown]

Thanks for providing this @rubentalstra.

Is the authorization header in the picture URL request based upon any actual need or standard/specification?
I'd be wary of sending the access token out, especially to other domains, but would prefer not to send at all if there's no need.

Also, I wouldn't look to have this run on each login, just the first registration/sync as per all existing details in support SSO options. We'll also need to add testing to cover the added functionality. I'm happy to make these changes/additions before merge, but I really just need to know about the authorization point above.

[rubentalstra]

@ssddanbrown hi, thank you for taking the time. The header is for sure needed if you use Microsoft because it will call a global endpoint. And based on the header it will return the users profile.

[ssddanbrown]

Thanks for this @rubentalstra.

I can confirm via testing.
Really not sure I want to go outside the spec just to suit Microsoft though.
Also, the origin between picture url and issuer is different so we can't put this behind a basic origin check.
Could have extra config options but that's getting dumb for a minor side features of an optional auth option.

I'm siding towards not sending any auth tokens, leaving any non-spec services like Entra/AzureAD use our logical theme hooks to work around their awkardness.

Another consideration for this PR: it's currently saving images as png, which probably will be the most common, but this is not assured; The spec does not confirm exact formats.