A distributed claim allows the OIDC provider to delegate a claim to a
separate URL. Distributed claims are of the form as seen below, and are
defined in the OIDC Connect Core 1.0, section 5.6.2.
See: https://openid.net/specs/openid-connect-core-1_0.html#AggregatedDistributedClaims
Example claim:
```
{
... (other normal claims)...
"_claim_names": {
"groups": "src1"
},
"_claim_sources": {
"src1": {
"endpoint": "https://www.example.com",
"access_token": "f005ba11"
},
},
}
```
Example response to a followup request to https://www.example.com is a
JWT-encoded claim token:
```
{
"iss": "https://www.example.com",
"aud": "my-client",
"groups": ["team1", "team2"],
"exp": 9876543210
}
```
Apart from the indirection, the distributed claim behaves exactly
the same as a standard claim. For Kubernetes, this means that the
token must be verified using the same approach as for the original OIDC
token. This requires the presence of "iss", "aud" and "exp" claims in
addition to "groups".
All existing OIDC options (e.g. groups prefix) apply.
Any claim can be made distributed, even though the "groups" claim is
the primary use case.
Allows groups to be a single string due to
https://github.com/kubernetes/kubernetes/issues/33290, even though
OIDC defines "groups" claim to be an array of strings. So, this will
be parsed correctly:
```
{
"iss": "https://www.example.com",
"aud": "my-client",
"groups": "team1",
"exp": 9876543210
}
```
Expects that distributed claims endpoints return JWT, per OIDC specs.
In case both a standard and a distributed claim with the same name
exist, standard claim wins. The specs seem undecided about the correct
approach here.
Distributed claims are resolved serially. This could be parallelized
for performance if needed.
Aggregated claims are silently skipped. Support could be added if
needed.
Kubernetes-commit: dfb527843ca1720ad64383fa5d6baea4113daa3e
fad0fdecfa