[RDBMS] Improvements to Management Plane for MySQL and PostgreSQL

[mjain2]

Description

This PR improves existing commands for the 'Flexible Servers' APIs within the RDBMS component.

Changes include:

• Update MySQL Connection string in create command
• Replace server name check with CheckNameAvailability API for MySQL & PostgreSQL
• Update help text and parameters for commands to be more descriptive and helpful to user (commands/arguments: vnet related, replica, firewall, restore). These were to improve customer experience.
• Update validators to use result from list-skus commands/API.
• Add command to enable user to connect to Flexible Server
• Enabled more tests for code coverage and command testing (validator & replica tests)
• Added new recording files with default locations for MySQL & PostgreSQL

Testing Guide

To run tests (these cover existing commands):
azdev test rdbms

New commands:
az mysql flexible-server connect --admin-user testuser --admin-passoword testpassword --name servername

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• The PR title and description has followed the guideline in Submitting Pull Requests.

• I adhere to the Command Guidelines.

[mjain2]

#15363 <-- new PR