Distributor: Move resource attribute promotion specialization to Overrides

[aknuds1]

What this PR does

Refactoring: Move resource attribute promotion specialization from distributor OTLP handler to validation.Overrides, for better abstraction of the pluggable logic. A couple of tests included. See comment for background.

Which issue(s) this PR fixes or relates to

Checklist

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

[colega]

I think this approach is still not great. We're just moving the if condition to a different implementation, instead of using polymorphism and injecting different implementation based on our needs.

All what OTLPHandler needs is this:

type OTelResourceAttributePromotionConfig interface {
	// PromoteOTelResourceAttributes returns which OTel resource attributes to promote for tenant ID.
	PromoteOTelResourceAttributes(id string) []string
}

You already have one implementation for that, which is Overrides. The OOP way of modifying the behavior of the application is not to modify the behavior of Overrides, is to inject a different implementation of OTelResourceAttributePromotionConfig to OTLPHandler on instantiation.