feat: support creating multi-source applications in New App panel

[dvcanton]

Implements #20964 based on Proposal #17108.

Depends on a fix #581 in the FormField component.

Side note: that is my first contribution to ArgoCD, therefore I am open to improvement ideas, suggestions and discussion points on this PR.

Checklist:

• [x ] 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.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• 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.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

[bunnyshell]

❗ Preview Environment deployment failed on Bunnyshell

See: Environment Details | Pipeline Logs

Available commands (reply to this comment):

• 🚀 /bns:deploy to redeploy the environment
• ❌ /bns:delete to remove the environment