Allow third-party filters to set the state for oidcClaimsQuery

[haveo]

Is your feature request related to a problem? Please describe.
The oidcClaimsQuery filter allows for arbitrary queries on claims using GJSON. Besides being able to be used for OIDC, other filters in Skipper that handle different authentication schemes also set the OIDC claims cache key allowing the oidcClaimsQuery filter to be called.
However, when working with a third-party filter, it is not possible to create a tokenContainer value because this type is private to the filters/auth package. Therefore it is not possible to set the OIDC claims cache key to a value that would be accepted by oidcClaimsQuery, since the filter expects a value of type tokenContainer to be stored there.
This prevents the reuse of oidcClaimsQuery when the authentication is done in a third-party filter.

Describe the solution you would like
A function should be exported that sets the OIDC claims cache key (given a map[string]interface{} claims value).

Describe alternatives you've considered (optional)
An alternative would be to export the tokenContainer type.

Would you like to work on it?
Yes

[szuecs]

Sounds great!
Please create a PR, thanks