refactor: make slack.Notifier public

[bsushmith]

• make slack.Notifier public

Changes for discussion - https://github.com/odpf/guardian/pull/370#discussion_r1125886870

[coveralls]

Pull Request Test Coverage Report for Build 4352351092

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.07%) to 75.029%

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Files with Coverage Reduction	New Missed Lines	%
plugins/notifiers/slack/client.go	4	70.83%
<!--	Total:	4		-->

Totals
Change from base Build 4340532339:	0.07%
Covered Lines:	7806
Relevant Lines:	10404

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💛 - Coveralls