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
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• Tests were added or are not required
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• Documentation was added or is not required

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[codecov]

Codecov Report

Attention: Patch coverage is 76.47059% with 16 lines in your changes missing coverage. Please review.

Project coverage is 67.65%. Comparing base (9383943) to head (adbc47d).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/grpcvtctldserver/server.go	75.00%	3 Missing ⚠️
go/vt/vtorc/logic/topology_recovery.go	0.00%	3 Missing ⚠️
go/vt/vtctl/vtctl.go	0.00%	2 Missing ⚠️
go/vt/vttablet/tabletmanager/rpc_backup.go	0.00%	2 Missing ⚠️
go/vt/wrangler/reparent.go	0.00%	2 Missing ⚠️
go/vt/wrangler/tablet.go	0.00%	2 Missing ⚠️
go/cmd/vtctldclient/command/keyspaces.go	66.66%	1 Missing ⚠️
go/vt/vtorc/inst/analysis_dao.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17448      +/-   ##
==========================================
- Coverage   67.67%   67.65%   -0.03%     
==========================================
  Files        1583     1584       +1     
  Lines      254363   254360       -3     
==========================================
- Hits       172140   172083      -57     
- Misses      82223    82277      +54     

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[timvaillancourt]

LGTM

Pointing out that users adding a custom policy (like us) will need to add a const to the new reparentutil/policy dir, in addition to our "shim" in reparentutil

[GuptaManan100]

@timvaillancourt No, not really. Your shim will just go into the policy package and thats all. You don't really need a constant to be defined. We're just doing that for more readability and easier navigation from tests.