Fetch the secret in only one function

[guillaumerose]

Changes

Interceptors all fetch the secret the same way. A util function exists
for this purpose interceptors.GetSecretToken. Let's use it for all
implementations.

http.Request is always nil when calling this function. It's something to
fix in the future.

With this single place where the code asks for secret, we can add a
good cache.

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

NONE

[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/bitbucket/bitbucket.go	92.9%	92.6%	-0.3
pkg/interceptors/github/github.go	93.3%	93.1%	-0.2
pkg/interceptors/gitlab/gitlab.go	92.9%	92.6%	-0.3

[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/bitbucket/bitbucket.go	92.9%	92.6%	-0.3
pkg/interceptors/github/github.go	93.3%	93.1%	-0.2
pkg/interceptors/gitlab/gitlab.go	92.9%	92.6%	-0.3

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: savitaashture

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[dibyom]

/lgtm

Thank you for the cleanup!

/kind cleanup