refactor: update subscription flow and update plugins

[mabdh]

related PRs raystack/proton#216 and raystack/salt#61

Changes

• Update create alerts to be more generic
• Simplify plugins
• Update subscription flow (remove transaction from subscription, sync config when adding namespace instead)
• Add end-to-end tests

[coveralls]

Pull Request Test Coverage Report for Build 3418019777

• 404 of 722 (55.96%) changed or added relevant lines in 29 files are covered.
• 7 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-3.7%) to 77.591%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/notification/service.go	6	7	85.71%
core/subscription/service.go	18	19	94.74%
core/template/template.go	0	1	0.0%
internal/api/v1beta1/receiver.go	11	12	91.67%
internal/store/postgres/alerts.go	4	5	80.0%
internal/store/postgres/namespace.go	15	17	88.24%
internal/store/postgres/postgres.go	7	9	77.78%
core/alert/service.go	17	21	80.95%
core/provider/service.go	5	9	55.56%
internal/api/v1beta1/subscription.go	21	25	84.0%

Files with Coverage Reduction	New Missed Lines	%
internal/api/v1beta1/alert.go	2	94.94%
plugins/providers/cortex/service.go	2	35.64%
plugins/receivers/slack/option.go	3	30.77%

Totals
Change from base Build 3374641907:	-3.7%
Covered Lines:	2770
Relevant Lines:	3570

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💛 - Coveralls