feat: update slack notification by adding block functionality to template

[utsav14nov]

feat: update slack notification by adding block functionality to template

[coveralls]

Pull Request Test Coverage Report for Build 4339427071

• 33 of 51 (64.71%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 74.962%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
plugins/notifiers/slack/client.go	26	44	59.09%

Totals
Change from base Build 4249606976:	-0.1%
Covered Lines:	7799
Relevant Lines:	10404

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💛 - Coveralls

[utsav14nov]

@rahmatrhd As checked in terraform, we are be able to read full content from a json file as string and use it as variable in config which makes easily maintainable and structured code in production.
And as you suggested we can add the default config as separate json in guardian and read it programmatically through code.
What say? @bsushmith @haveiss

[bsushmith]

@utsav14nov Yes. That is fine from a deployment using terraform. But for other kinds of deployment, @rahmatrhd's comment still stands true.
But then again, it should be fine, as long as the user is able to maintain these big strings on their end irrespective of deployment method.

[singhvikash11]

@rahmatrhd please have a look of pr #330