Add a RegisterInterceptor helper function

[dibyom]

Changes

This makes it a lot easier to write custom interceptors while utilizing the
rest of the server helpers that we have created for the packaged core
interceptors.

(Since the interceptors field within Server is not exported, it becomes impossible to use the server code for interceptors not written within the Trigger source tree)

This should help a lot in the process of migrating the dogfooding interceptors in https://github.com/tektoncd/plumbing/tree/main/tekton/ci/interceptors to use the ClusterInterceptor CRD

/cc @afrittoli

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• Includes tests (if functionality changed/added)
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Release Notes

Adds a RegisterInterceptor helper function that can be used by authors of custom interceptors to utilize the same interceptor server framework that we use for the core interceptors. See https://github.com/tektoncd/plumbing/pull/967 for an example.

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-triggers-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/interceptors/server/server.go	82.5%	83.7%	1.2

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-triggers-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/interceptors/server/server.go	82.5%	83.7%	1.2

[savitaashture]

Looks good to me 👍

Could you add release note about the RegisterInterceptor.

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: savitaashture

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[dibyom]

Could you add release note about the RegisterInterceptor.

Done

[savitaashture]

/lgtm