Add automated tests for webhooks extension

[a-roberts]

Like we do for the dashboard and pipeline repositories, we need to be able to build and test PRs automatically for code added into the webhooks-extension folder.

This includes running the Go tests and web tests when we eventually add them.

A good place to start is to look at the code here and here.

[vdemeester]

/assign

[a-roberts]

@vdemeester hero! 😄

/meow

[tekton-robot]

@a-roberts:

In response to this:

@vdemeester hero! 😄

/meow

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