oidcclaimquery: better error handling for casting

[MustafaSaber]

Describe the bug

if oidcclaimsquery get a jwt token that doesn't have sub for any reason it will fail to cast nil to string.

skipper/filters/auth/oidc_introspection.go

Line 134 in 00ff742

sub := token.Claims["sub"].(string)

To Reproduce

use a token without sub

Expected behavior

Better error handling instead of failing without clue what is happening.

[wassafshahzad]

I would like to pick this up

[AlexanderYastrebov]

sub claim is optional per https://datatracker.ietf.org/doc/html/rfc7519#section-4.1.2

The question is what to use as user id when sub claim is not present.

[wassafshahzad]

sub claim is optional per https://datatracker.ietf.org/doc/html/rfc7519#section-4.1.2

The question is what to use as user id when sub claim is not present.

@AlexanderYastrebov
I apologize as I might not be fully aware of the context but do we have any other data that might be suitable in the JWT and if that is not possible we could panic for now and identify the correct key later .
I looked a but deeper there is a UserInfo struct and maybe we can use the username from there

type UserInfo struct {
	Subject       string `json:"sub"`
	Profile       string `json:"profile"`
	Email         string `json:"email"`
	EmailVerified bool   `json:"email_verified"`

	claims []byte
}