Add support for Git Repository dry run flag

[bender100]

Would like the ability to show flux diffs in pull requests. I'm new to flux and my understanding is we would have to deploy our own infrastructure to support this use case however it would be useful if Git Repository source had a parameter to specify a branch pattern and if branch matched the pattern it would only do a dry run. This would facilitate having pull requests use a pattern like wip-* or feature-* which would trigger a dry run or flux kustomization diff and then using notification controller we could publish the results of the diff back to the PR but without applying any of the changes. After merging to main for example then it would no longer match the branch pattern for dry run so it would reconcile the changes and apply to the clusters. I've included example below. This solution uses existing workflow so easily scales to handle organizations that operate large numbers of clusters without the need to come up with bespoke solution to handle what i think would be a common use case and best practice. Would like to hear if this is feasible or if there is a better approach to solving this problem. Thank you.

---

apiVersion: source.toolkit.fluxcd.io/v1
kind: GitRepository
metadata:
name: podinfo
namespace: default
spec:
interval: 5m0s
url: https://github.com/stefanprodan/podinfo
ref:
branch: main
dryRun: feature-*