feat: add Init before AddInstance while backing up

[mlycore]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What this PR does / why we need it:

Using gs_probackup init to init dn-backup-path before AddInstance

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?

[sonarqubecloud]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 36.81%. Comparing base (3604a24) to head (8cb6480).
⚠️ Report is 257 commits behind head on main.

Files with missing lines	Patch %	Lines
pitr/agent/internal/handler/backup.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #480      +/-   ##
==========================================
- Coverage   39.71%   36.81%   -2.90%     
==========================================
  Files          51       60       +9     
  Lines        6083     6945     +862     
==========================================
+ Hits         2416     2557     +141     
- Misses       3448     4142     +694     
- Partials      219      246      +27     

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