Provide a library for OIDC token management

[creydr]

As the Eventing OIDC feature track describes, we need a library to take care about the OIDC token management.

This library should include the following:

• Request a new OIDC token for a given SA (e.g. from status.auth.serviceAccountName) and for the targets Audience (.status.address.audience).
• Cache tokens and take care about renewals