Make all durability policy names constants

[GuptaManan100]

Description

This PR makes the durability policy names constants and uses them everywhere. This makes the code more clean.

Related Issue(s)

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

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.67%. Comparing base (9383943) to head (76f7e08).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/vtctl.go	0.00%	2 Missing ⚠️
go/cmd/vtctldclient/command/keyspaces.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17448      +/-   ##
==========================================
- Coverage   67.67%   67.67%   -0.01%     
==========================================
  Files        1583     1584       +1     
  Lines      254363   254386      +23     
==========================================
+ Hits       172140   172151      +11     
- Misses      82223    82235      +12     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[dbussink]

@GuptaManan100 Is this a lightweight package so that we don't end up including a large dependency chain in all places we add this to? That could increase potentially binary and / or compile times or makes parts harder to use as a library (if we do that for any of these bits really).

In the past I've always checked this and also sometimes explicitly opted for a new package then just with the contants to mitigate problems mentioned above.