Feat: add support for secret provider for webhook

[PeterSurda]

Webhook secret can how use buildbot's secret management interface.

e.g.

c['www']['change_hook_dialects'] = {
    'gitea': {
        'secret': util.Secret('my_gitea_webhook_secret'),
        'class': GiteaHandler,
        'onlyIncludePushCommit': True,
    }
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Used GitHub buildbot plugin as a reference.

[PeterSurda]

any feedback?

[pampersrocker]

Sorry, I'm quite busy at the moment. You can verify it by running the trial Unit tests against it and if you feel comfortable create a unit test with an active secret as well.

[PeterSurda]

Don't know how to run the tests. travis-ci gets stuck in a loop and times out: https://travis-ci.org/github/PeterSurda/buildbot-gitea . The code works on my system.

[pampersrocker]

Thank you for the contribution! I've added unit tests for a secret provider. This will be part of the next minor release, but I want another feature done first before making another release.

I know the travis-ci gets stuck in some dependency setup but was not able to figure out how to solve it.