feat(storage): thread new storage reference through all read requests

[GeorgeMac]

Fixes: FLI-724

This is a combination refactor and addition of a new reference field on all read requests.

In order to thread this everywhere with an empty default, I packaged up many of the storage method call arguments into structures. In particular, I created a few common structures.

NamespaceRequest is used for operations which only need a namespace:

flipt/internal/storage/storage.go

Lines 373 to 378 in a936863

	// NamespaceRequest is used to identify a request predicated by both a revision and a namespace.
	// This is used to identify namespaces and list resources directly beneath them (e.g. flags and segments).
	type NamespaceRequest struct {
	ReferenceRequest
	key string
	}

ResourceRequest is used for any operation which is scoped to some resource (e.g. scoped by flag or segment):

flipt/internal/storage/storage.go

Lines 408 to 413 in a936863

	// ResourceRequest is used to identify a request predicated by revision, namespace and a key.
	// This is used for core resources (e.g. flag and segment) as well as to list sub-resources (e.g. rules and constraints).
	type ResourceRequest struct {
	NamespaceRequest
	Key string
	}

Each of these types embed a reference which can be empty (zero state).
In a subsequent PR I will start to depend upon these references.
However, the server layer has been updated in this change to start threading these references from the RPC layer into the storage layer.

[codecov]

Codecov Report

Attention: 25 lines in your changes are missing coverage. Please review.

Comparison is base (5aa70ec) 71.57% compared to head (2f33b1d) 71.37%.

Files	Patch %	Lines
internal/storage/fs/snapshot.go	82.14%	9 Missing and 1 partial ⚠️
internal/server/evaluator.go	50.00%	8 Missing ⚠️
internal/storage/fs/store.go	88.70%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2570      +/-   ##
==========================================
- Coverage   71.57%   71.37%   -0.21%     
==========================================
  Files          84       84              
  Lines        8085     8013      -72     
==========================================
- Hits         5787     5719      -68     
- Misses       1947     1949       +2     
+ Partials      351      345       -6     

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[markphelps]

amazing refactor!!

[GeorgeMac]

@markphelps I just renamed a bunch of these new things and added some additional doc comments.
I mostly renamed things from "predicate" to "request" as I felt it was too unecessarilly fancy talk 🎩 .

[GeorgeMac]

Also, this last commit allows the existing fs stores to skip taking a reference and ignoring it for now.
Instead, there is an adaptor that does the dropping reference part and log if its non-empty.
The Git and OCI stores will ultimately implement the ReferencedSnapshotStore interface and support multiple references that way. But the others will warn if you try to access no default references through the adaptor.

[markphelps]

wicked

[GeorgeMac]

Last pass @markphelps

I rebased and fixed this up after the the GCS changes and the cache changes 👍

I've updated the PR

[yquansah]

This looks good to me! Nice work.

[yquansah]

Nice abstractions!