feat(validate): add ability to further constrain schema

[GeorgeMac]

This is in response to a discussion had with our friends at Paradigm.

The request was:

Hi team, given we have flipt validate is there perhaps a way to force some fields to be mandatory?
In our case description is required or we won't have people  outside of developers knowing what that flag actually does.

We leverage CUE under the hood to power flipt validate and this is driven by our flipt feature CUE schema:

flipt/internal/cue/flipt.cue

Lines 1 to 20 in de98001

	close({
	version: "1.0" | "1.1" | *"1.2"
	namespace: string & =~"^[-_,A-Za-z0-9]+$" | *"default"
	flags: [...{_version: version} & #Flag]
	segments: [...#Segment]
	})
	#Flag: {
	_version: string
	key: string & =~"^[-_,A-Za-z0-9]+$"
	name: string & =~"^.+$"
	description?: string
	enabled: bool | *false
	variants: [...#Variant]
	rules: [...#Rule]
	if _version == "1.1" || _version == "1.2" {
	type: "BOOLEAN_FLAG_TYPE" | *"VARIANT_FLAG_TYPE"
	#FlagBoolean | *{}
	}
	}

This proposed change adds the ability to extend this schema with your own valid subset of this schema.

flipt validate -e extended.cue features.json

Taking the original request to be able to make e.g. description required, this could be achieved with the following extensions to the base schema (in extended.cue):

#Flag: {
	description: =~"^.+$"
}

This adds to the existing #Flag definition and says that description is required (drops the ?) and that is must be a non-empty string (matches the regexp =~"^.+$").
Taking the following JSON example in a file named features.json:

{
  "flags": [{"key": "foo", "name": "Foo"}]
}

Without the extended CUE definition, flipt validate will pass:

➜  flipt validate features.json
➜  echo $?
0

However, by passing in the extended CUE definition via -e (short for --extra-schema) we can add the restrictions:

➜  flipt validate -e extended.cue features.json
Validation failed!

- Message  : flags.0.description: incomplete value =~"^.+$"
  File     : features.json
  Line     : 2

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (de98001) 72.99% compared to head (6c14055) 72.87%.

Files	Patch %	Lines
internal/cue/validate.go	29.41%	11 Missing and 1 partial ⚠️
internal/storage/fs/snapshot.go	75.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2677      +/-   ##
==========================================
- Coverage   72.99%   72.87%   -0.13%     
==========================================
  Files          85       85              
  Lines        8325     8346      +21     
==========================================
+ Hits         6077     6082       +5     
- Misses       1871     1886      +15     
- Partials      377      378       +1     

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[markphelps]

Nice! Would you mind cue'ing up (haha) a docs PR?

[GeorgeMac]

I see what you did there

[markphelps]

@GeorgeMac there seems to be a slight bug in nightly where it doesn't report the line number accurately