Add ability to configure event driven functions

Implement an event_trigger variable to enable deployment of event driven functions. Also, fix an issue with the project data source requiring the project_id attribute to be specified in certain circumstances.

BREAKING CHANGE: Changing the trigger_http variable's default from true to null is required to allow us to switch between creating http triggered functions and event driven functions. However, this could cause functions which are currently deployed using this module to be destroyed. Therefore, we will bump this module up a major version to v2.0.0 once this MR is merged.

Also, the use of optional variable values in the new event_trigger variable requires Terraform >= 1.3 which has been defined in the versions.tf. This is also a breaking change.

Part of #3 (closed) and https://gitlab.developers.cam.ac.uk/uis/devops/infra/gcp-product-factory/-/issues/58 (for the MS Teams function bit).