make xtrabackup ShouldDrainForBackup configurable

[rvrangel]

Description

This will let the operator choose if they wish to drain a tablet when taking an xtrabackup as discussed in #18430

Related Issue(s)

Fixes #18430

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.48%. Comparing base (4fbab4f) to head (39323cc).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18431      +/-   ##
==========================================
- Coverage   67.49%   67.48%   -0.02%     
==========================================
  Files        1607     1607              
  Lines      262741   262772      +31     
==========================================
- Hits       177339   177329      -10     
- Misses      85402    85443      +41     

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