Add the ability to filter MRs by 'state' to Gitlab PR Generator

[Aym3nTN]

Summary

In the Gitlab PR Generators, I wanted to give the developer the ability to provide the 'state' (usually opened) of Gitlab merge requests he wants to create an ephemeral environment for.

Motivation

In the current Gitlab PR Gen, it grabs all the MRs even those with state 'closed' or 'merged'. We wanted to limit it to only 'opened' MRs so when the MRs gets closed or merged the ephemeral environment dies.

Proposal

This should be implemented by simply adding an attribute like pullRequestState to the GitlabPullRequestGenerator with the state that the developer wants to track.
This enhancement proposal is only for Gitlab PR Gen since each SCM provider has its own way to filter MRs or PRs and its own state naming style.