Add matcher for used OIDC user name

[creydr]

Add a event matcher, to check the OIDC user name of the received event.

This adds two new functions:

• MatchOIDCUser(username) to directly check if the given username matches the OIDC username in EventInfo
• MatchOIDCUserFromResource(gvr, resourceName) to check, if the OIDC username from the given resource matches the OIDC username in the EventInfo

This allows e.g. testcases like the following:

...
f.Alpha("Broker").
	Must("subscriber receives event from correct sender identity", eventassert.OnStore(sink).MatchWithContext(eventassert.MatchOIDCUserFromResource(triggerresources.GVR(), triggerName)).Exact(1))

[creydr]

/cc @pierDipi @Cali0707
ready for review

[Cali0707]

Just some small nits that I think will make it easier to debug failures when using this, otherwise LGTM

[Cali0707]

/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Cali0707, creydr

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [Cali0707,creydr]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment