[FEATURE]: Improve contributor workflow: Introduce “Awaiting Maintainer Approval” status for new issues

### Why do we need this improvement?

@derberg @Adi-204 Currently, for many new issues in AsyncAPI Generator, contributors often start working immediately and open a PR. In several cases, maintainers later close these PRs with feedback like “Please wait for maintainer approval before working on this”.

At the same time, there are situations where contributors start working on an issue weeks later without explicit approval and those contributions are accepted. This creates an inconsistent contributor experience and uncertainty around when it is actually okay to start working on an issue.

Problem

- Contributors don’t have a clear signal on when an issue is approved for work
- PRs are sometimes closed after effort is already spent
- Approval expectations vary depending on timing and reviewer availability
- This discourages new contributors and leads to wasted effort

### How will this change help?

Introduce a lightweight but explicit approval flow:

Automatically label all new issues as Awaiting Maintainer Approval
Maintainers review the issue and:
Add an Approved (or Ready for Contribution) label
or
Assign the issue to a contributor

Contributors are encouraged to start working only after approval/assignment
This would:

- Set clear expectations for contributors
- Prevent premature PRs
- Reduce back-and-forth and PR closures
- Make the contribution process more consistent and contributor-friendly

### Screenshots

### How could it be implemented/designed?

Add this rule to CONTRIBUTING.md
Use GitHub Issue Templates or Actions to auto-apply the label
Mention explicitly: “Please wait for maintainer approval before starting work”

### 🚧 Breaking changes

Yes

### 👀 Have you checked for similar open issues?

- [x] I checked and didn't find a similar issue

### 🏢 Have you read the Contributing Guidelines?

- [x] I have read the [Contributing Guidelines](https://github.com/asyncapi/.github/blob/master/CONTRIBUTING.md)

### Are you willing to work on this issue?

None

Start by reading the linked CONTRIBUTING.md guidance and reviewing the repository’s issue templates or GitHub Actions configuration, if present. Define how new issues receive an Awaiting Maintainer Approval label and how maintainers mark approval or assignment; done means the workflow and contributor instructions consistently communicate when work may begin.

Written by the indexing model from the issue text.