Add CIMD support for automatic oauth app creation, and PKCE support

[coyotte508]

Add CIMD support, for automatic oauth app creation, and add PKCE support

Just need to set OPENID_CLIENT_ID=__CIMD__ or OPENID_CONFIG={CLIENT_ID:"__CIMD__",...}

By the way after this PR, no need for OPENID_CLIENT_SECRET, so open id config can be a public var and does not need to be secret (just remove the secret from it), cc @rtrompier

Internal ref: https://github.com/huggingface-internal/moon-landing/pull/15624

Tested on https://huggingface.co/spaces/coyotte508/chat-ui - first real use of CIMD cc @evalstate :)

Note

• new cookie hfChat-codeVerifier (lasts 1 hour) during oauth process
• we could probably have set the cookies' paths to /${base} instead of "/"... (too late to change it?)

[coyotte508]

instead of forwarding url everywhere, we could use contextStore like here: https://github.com/huggingface/Mongoku/blob/2a6c715c5bf4a7f7351aa3b3ce90ecd857e61d70/src/hooks.server.ts#L17-L48

(not sure for compat with non-node runtimes)

could also be used for structued logs (eg request id, ...)