Sec. index: make compound filters utillize sec. indexes

[islamaliev]

at the moment tests like this don't use indexes:

func TestJSONIndex_WithCompoundFilterCondition_ShouldUseIndex(t *testing.T) {
	req := `query {
		User(filter: {_and: [
			{custom: {height: {_eq: 180}}},
			{custom: {weight: {_eq: 80}}}
		]}) {
			name
		}
	}`
	test := testUtils.TestCase{
		Actions: []any{
			testUtils.SchemaUpdate{
				Schema: `
					type User {
						name: String 
						custom: JSON @index
					}`,
			},
			testUtils.CreateDoc{
				Doc: `{
					"name": "John",
					"custom": {"height": 168, "weight": 70}
				}`,
			},
			testUtils.CreateDoc{
				Doc: `{
					"name": "Islam",
					"custom": {"height": 180, "weight": 80}
				}`,
			},
			testUtils.CreateDoc{
				Doc: `{
					"name": "Shahzad",
					"custom": {"height": 180, "weight": 75}
				}`,
			},
			testUtils.CreateDoc{
				Doc: `{
					"name": "Keenan",
					"custom": {"height": 190, "weight": 85}
				}`,
			},
			testUtils.Request{
				Request: req,
				Results: map[string]any{
					"User": []map[string]any{
						{"name": "Islam"},
					},
				},
			},
			testUtils.Request{
				Request:  makeExplainQuery(req),
				Asserter: testUtils.NewExplainAsserter().WithIndexFetches(2),
			},
		},
	}

	testUtils.ExecuteTestCase(t, test)
}

The test uses json, but the behaviour should be the same for other field types.
Most likely majority of the work will be in function findIndexByFilteringField in internal/planner/select.go. But we should add a bunch of tests as well.