feat(appset): add key title to PR generator (#16447)

[ggjulio]

Closes #16447

Change

Add a new key title for all SCMaaS providers on generator pullRequest.

Why

Adding the title allow to filter PRs by title.
Which is a must for SCMaaS like bitbucket that do not support labels.
(branch filtering is not an alternative to labels because branch names are "immutable")

Example (see crenshaw-dev's comment #16447 (comment) ):

apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: preview
  namespace: argocd
spec:
  goTemplate: true
  goTemplateOptions: ["missingkey=error"]
  generators:
  - matrix:
      generators:
      - pullRequest:
            bitbucketServer:
              project: project-name
              repo: repo-name
              api: https://bitbucket.example.com
              basicAuth:
                // ...
       - list:
            elements:
            - titleMatches: '{{ regexMatch ".*[PREVIEW].*" .title }}'
    selector:
      matchLabels:
       titleMatches: "true"
template:
  // ...

---

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.