Add typing for json_reporter and sub-classes

[DanielNoord]

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Type of Changes

	Type
✓	🔨 Refactoring

Description

Another instalment of breaking up #4954.
Although the number of changed lines is quite high, it is basically adding typing to the same methods in a bunch of different (sub)classes.

[coveralls]

Pull Request Test Coverage Report for Build 1243056352

• 25 of 31 (80.65%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 93.053%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pylint/reporters/collecting_reporter.py	2	3	66.67%
pylint/testutils/reporter_for_tests.py	6	8	75.0%
pylint/reporters/base_reporter.py	5	8	62.5%

Totals
Change from base Build 1242365285:	-0.03%
Covered Lines:	13315
Relevant Lines:	14309

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💛 - Coveralls