[EPIC] Design End-to-End 3rd Party Extension Submission Process

## Description

Design and implement a comprehensive end-to-end process that enables 3rd party extension authors to submit their extensions to the azd extension registry. This includes establishing quality gates, security requirements, privacy guidelines, compliance requirements, and telemetry standards.

## Acceptance Criteria

- Documented submission process for 3rd party authors
- Automated quality gate pipeline for extension validation
- Security requirements clearly defined and verified
- Privacy requirements documented and enforced
- Telemetry requirements established
- CELA compliance guidance provided
- Authors can use `azd x` developer extension to submit
- Clear documentation on extension certification levels

## Child Issues

- [ ] #6218 Automated Quality Gates Pipeline for Extensions
- [ ] #6219 Leverage azd x for Extension Submission
- [ ] #6220 Identify CELA Guidance for 3rd Party Extensions
- [ ] #6221 Define Security Requirements for 3rd Party Extensions
- [ ] #6222 Define Privacy Requirements for 3rd Party Extensions
- [ ] #6223 Define Telemetry Requirements for 3rd Party Extensions
- [ ] #6224 Automated Duplicate Detection for Extension Submissions

## Related Issues

- #5768 (Move dev extension registry to main branch) - Infrastructure prerequisite
- #5229 (Write up process on new extensions) - Partial overlap, should align

## Benefits

- Establishes trust in 3rd party extensions
- Reduces security/compliance risk
- Clear expectations for extension authors
- Enables ecosystem growth beyond Microsoft-authored extensions
- Protects users through quality and security gates