Move validate_custom_event validators into validators directory

[lrafeei]

Overview

This PR moves validate_custom_event_in_application_stats_engine and validate_custom_event_count into its own separate file within the testing_support/validators directory

[github-actions[bot]]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	24	0	0	2.64s
✅ PYTHON	flake8	24		0	1.56s
✅ PYTHON	isort	24	0	0	0.27s
✅ PYTHON	pylint	24		0	10.86s

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