Add ability to support strict JSON unmarshal for limits

[kavirajk]

What this PR does:
Main problem is currently we cannot strict unmarshal JSON for Limits struct using json.DisallowUnknownFields(). Because custom UnmarshalJSON uses simple json.Unmarshal() function. (is it intentional?)

This PR Adds ability to support strict JSON unmarshal for limits struct

This is the default behaviour for YAML now.

	in := `{"unknown_fields": 100}`
	l := validation.Limits{}

	fmt.Println(yaml.UnmarshalStrict([]byte(in), &l))

	// yaml: unmarshal errors:
	// line 1: field unknown_fields not found in type validation.plain

This PR adds same behaviour if unmarshalled from JSON input as well.

Which issue(s) this PR fixes:
Fixes NA

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[stevesg]

Looks fine assuming this is the behavior we want, but some points to consider:

• Does mean that we lose forward compatibility? i.e. a "newer" configuration file (with unknown fields) cannot be parsed by an older version of Cortex. Whatever is producing the limits/overrides files, must be perfectly in sync with the Cortex version. This may be an acceptable trade-off.
• Correct me if I'm wrong, but does this now mean that JSON parsing does not accept unknown fields, whilst the YAML parsing does? This perhaps is a little odd, I would expect both to have the same behavior.
• Also [unrelated curiosity], given runtime_config.go:65, I wonder if it was actually intentional for the YAML parsing to accept unknown fields...

[pstibrany]

• Correct me if I'm wrong, but does this now mean that JSON parsing does not accept unknown fields, whilst the YAML parsing does? This perhaps is a little odd, I would expect both to have the same behavior.

As you point out in your next point, we already use strict parsing for YAML runtime config (which includes overrides), so this PR actually unifies both YAML and JSON parsing to strict.

Cortex itself doesn't use JSON parsing for limits, as far as I know.

[stevesg]

Ah my mistake, I misread the UnmarshalYAML function. OK then it makes sense that both are strict.

[kavirajk]

Thanks @stevesg @pstibrany!. Yes that's right this PR unifies both JSON and YAML strict with unknown fields!

[pracucci]

LGTM!

[bboreham]

Great job!