feat: Add support to explain countNode attributes.

[shahzadlone]

RELEVANT ISSUE(S)

Resolves #478

DESCRIPTION

• Adds ability to explain attributes of countNode.
• Fixes a bug which was omitting the aggregate nodes.

Request:

query @explain {
	author {
		name
		numberOfBooks: _count(books: {})
	}
}

Response:

{
	"explain": {
		"selectTopNode": {
			"countNode": {
				"filter":         nil,
				"sourceProperty": "books",
				"selectNode": {
					"filter": nil,
					"typeIndexJoin": {
						"scanNode": {
							"collectionID":   "3",
							"collectionName": "author",
							"filter":         nil,
							"spans": []{
								{
									"start": "/3",
									"end":   "/4",
								}
							}
						}
					}
				}
			}
		}
	}
}

HOW HAS THIS BEEN TESTED?

Integration tests.

CHECKLIST:

• I have commented the code, particularly in hard-to-understand areas.
• I have made sure that the PR title adheres to the conventional commit style (subset of the ones we use can be found under: tools/configs/chglog/config.yml

ENVIRONMENT / OS THIS WAS TESTED ON?

Please specify which of the following was this tested on (remove or add your own):

• Arch Linux

[AndrewSisley]

Looks good. Left an optional suggestion on selectTN.source

[jsimnz]

Straight forward, minor suggestion. LGTM

[codecov]

Codecov Report

Merging #504 (6ebd9a5) into develop (b8beabb) will increase coverage by 0.10%.
The diff coverage is 94.44%.

@@             Coverage Diff             @@
##           develop     #504      +/-   ##
===========================================
+ Coverage    54.10%   54.20%   +0.10%     
===========================================
  Files           97       97              
  Lines        13099    13109      +10     
===========================================
+ Hits          7087     7106      +19     
+ Misses        5337     5328       -9     
  Partials       675      675              

Impacted Files	Coverage Δ
query/graphql/planner/planner.go	75.00% <66.66%> (ø)
query/graphql/planner/count.go	88.88% <100.00%> (+11.53%)	⬆️
query/graphql/planner/select.go	75.66% <100.00%> (ø)
query/graphql/planner/type_join.go	69.07% <100.00%> (ø)
query/graphql/planner/render.go	85.26% <0.00%> (+1.05%)	⬆️
query/graphql/planner/sort.go	75.67% <0.00%> (+1.35%)	⬆️
query/graphql/planner/limit.go	81.25% <0.00%> (+3.12%)	⬆️