953 - Adding possibility of using "countDistinct" in aggregation

[piotrkosecki]

This is a draft which possibly resolves #953

It introduces change which adds possibility of using distinct with aggregation functions.

Example query can look like that:

{
	"fields": ["meta_voting_period", "meta_cycle", "level"],
	"predicates": [{
		"field": "timestamp",
		"operation": "after",
		"set": [1577854800000],
		"inverse": false
	}],
	"orderBy": [{
		"field": "meta_voting_period",
		"direction": "desc"
	}],
	"aggregation": [{
		"field": "meta_cycle",
		"function": "countDistinct"
	}, {
		"field": "level",
		"function": "count"
	}],
	"limit": 1000
}

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
0.0% Duplication

[vishakh]

Please add an example to the tutorial on the Github wiki.

[vishakh]

Note to self: This fixes a compilation issue.