Add MySQL backup, and full export for Mongo when no query.

[ZhangYongguang5467]

Description

This PR adds MySQL external backup support, improves table discovery, and adjusts Mongo export behavior:

1. MySQL backup via external commands (single table and merged tables)
2. Regex table discovery supports MySQL
3. MongoDB: if no query is provided, export all data (no longer skip)
4. Runtime image includes mysql-client

How Has This Been Tested?

• Verified that the tool correctly detects when a target table does not exist.
• Tested automatic creation of target tables with the correct schema.
• Confirmed that indexes are created on the target tables as specified.
• Ensured that data synchronization occurs accurately after table and index creation.

[codecov]

Codecov Report

❌ Patch coverage is 26.78133% with 298 lines in your changes missing coverage. Please review.
✅ Project coverage is 41.62%. Comparing base (f439cf1) to head (ec544da).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/backup/mysql.go	20.82%	270 Missing ⚠️
pkg/backup/executor.go	0.00%	20 Missing ⚠️
pkg/utils/gcs_upload.go	33.33%	4 Missing ⚠️
pkg/backup/mysql_test_helpers.go	94.59%	2 Missing ⚠️
pkg/syncer/mongodb/mongodb.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   40.12%   41.62%   +1.50%     
==========================================
  Files          37       39       +2     
  Lines       10907     8940    -1967     
==========================================
- Hits         4376     3721     -655     
+ Misses       6066     4752    -1314     
- Partials      465      467       +2     

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