feat: add MySQL command support

[enbiyagoral]

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This PR adds MySQL connection support to the Boundary CLI. Users can now securely and easily connect to MySQL targets using the boundary connect mysql command.

What’s Added?

• boundary connect mysql command
• Credential and target support for MySQL
• Required tests and documentation updates

Related Issue

Closes #5740

Testing

• [x] All tests passed successfully
• [x] MySQL connection tests were performed

Images

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Note: This PR follows the same architecture and UX as the existing helper command for PostgreSQL.