Leverage azd x for Extension Submission

## Description

Add `azd x submit` command to enable extension authors to submit their extensions to the registry with proper versioning, validation, and status tracking.

## Acceptance Criteria

- `azd x submit` command implemented
- Integration with automated quality gates pipeline
- Versioning validation (semantic versioning compliance)
- Changelog validation and requirements
- Submission status tracking (pending, in-review, approved, rejected)
- Support for submitting updates to existing extensions
- Clear error messages for validation failures
- Documentation for extension authors on submission process

## Implementation Notes

The submission command should:
- Validate extension package before submission
- Trigger quality gates pipeline automatically
- Provide progress updates during submission
- Return submission ID for tracking
- Support dry-run mode for testing
- Handle authentication/authorization for submitters

## Parent Issue

This is part of the 3rd Party Extension Submission Process epic.

## Labels

extensions, ext-x, feature