Update filedb cursor implementation

[khanhtc1202]

What this PR does / why we need it:

Currently, in order to use the datastore List interface in models' stores, Iterator.Cursor interface must be implemented (for instance: applicationstore). We're not required filedb to have Cursor implementation for now so this PR change to implementation for it from return error to dummy cursor (empty string).

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[pipecd-bot]

The following ISSUES will be created once got merged. If you want me to skip creating the issue, you can use /todo skip command.

Details

---

1. Implement cursor for filedb datastore.

pipecd/pkg/datastore/filedb/iterator.go

Lines 46 to 48 in f935fbc

	// TODO: Implement cursor for filedb datastore.
	return "", nil
	}

This was created by todo plugin since "TODO:" was found in f935fbc when #3459 was merged. cc: @khanhtc1202.

[pipecd-bot]

Code coverage for golang is 34.94%. This pull request increases coverage by 0.03%.

File	Function	Base	Head	Diff
pkg/config/control_plane.go	ControlPlaneDataStore.UnmarshalJSON	64.29%	60.00%	-4.29%
pkg/datastore/filedb/iterator.go	Iterator.Next	0.00%	100.00%	+100.00%
pkg/app/piped/executor/cloudrun/deploy.go	deployExecutor.ensurePromote	0.00%	0.00%	+0.00%
pkg/app/piped/executor/cloudrun/rollback.go	rollbackExecutor.ensureRollback	0.00%	0.00%	+0.00%

[knanao]

Nice
/lgtm

[nghialv]

Got it.

/approve

[pipecd-bot]

This pull request is APPROVED by nghialv.

Approvers can cancel the approval by writing /approve cancel in a comment. Any additional commits also will change this pull request to be not-approved.