Add specific pipelines configuration for Pull Request events

Pipelines are triggered by push. There is the possibility to configure a number of other webhooks, e.g. 'on pull request'. However it is not possible to connect them with a pipeline and disable or reconfigure the default 'Pipelines' webhook which triggers on push.

If it were possible to have different webhooks as triggers and distinguish between them inside the pipeline it would enable:

• to configure different pipelines for different events (webhooks), e.g.:

 - just unit tests with fast feedback loop for every change on feature branch,

 - heavy, time consuming quality assurance for pull request;

• better integration with "Sonar for Bitbucket Cloud", especially with respect to the "Shows Sonar code issues in pull requests" feature