Add pre-freeze and post-thaw scripts for volume-based backups using BACKUP STAGE

Overview
Add official pre-freeze and post-thaw scripts to enable consistent volume-based backups of MariaDB using storage snapshots (Veeam, Commvault, etc.) with the BACKUP STAGE commands.

Motivation
Many organizations use enterprise backup solutions that rely on storage-level snapshots. Currently, there are no official scripts to integrate MariaDB's BACKUP STAGE functionality with these backup products. Users either:

• Use FLUSH TABLES WITH READ LOCK (FTWRL), which blocks all reads
• Use mariadb-backup, which requires extra disk space, I/O, and more complex recovery procedures
• Take inconsistent snapshots

Proposed Solution
Provide scripts that:

• Pre-freeze script: Executes BACKUP STAGE START → BACKUP STAGE BLOCK_COMMIT before the snapshot
• Post-thaw script: Executes BACKUP STAGE END after the snapshot completes
• Maintain the same database session across both scripts (required for BACKUP STAGE to work)
• Include proper error handling and logging
• Support configuration via command-line parameters or configuration file

Key Advantages

• No read locks on the database (unlike FTWRL)
• Very brief write lock (only during BLOCK_COMMIT)
• No extra disk space required on the database server
• No additional I/O during backup operations
• Simple recovery: Just restore the VM snapshot and start MariaDB
• Minimal impact on production workloads

Technical Challenges
The BACKUP STAGE commands are session-based, requiring the database connection to remain alive between the pre-freeze and post-thaw scripts. This makes the implementation more complex than the traditional FTWRL approach.

Use Case
Organizations with:

• Large databases across multiple VMs
• Enterprise backup solutions (Veeam, Commvault, etc.)
• Limited disk space for staging backups
• Need for minimal backup impact on production
• Infrequent, controlled DDL operations

Discussions in:

• Zulip: https://mariadb.zulipchat.com/#narrow/channel/118759-general/topic/Pre-freeze.2FPost-thaw.20Scripts.20for.20Volume-based.20Backups/with/575995563
• discussion mailing list: https://lists.mariadb.org/hyperkitty/list/discuss@lists.mariadb.org/thread/6DPFOTQ42SOVYBRXU36JZQ3A5OOOMRJI/