refactor: move receivers and providers into plugins

[mabdh]

Changes

• Move receivers and providers as plugins
• Change behaviour of upsert rules for cortex
  • Previously
    • Upsert rule API -> write to DB -> get all rules in a rule group (collection of rules) of a tenant from DB -> write rule group to cortex
  • Now
    • Upsert rule API -> write to DB -> get a rule group from cortex -> upsert the rule to the fetched rule group -> write rule group to cortex
• When API return 500, hide the error details and log the error detail instead
• Add end-to-end API test for cortex
• Simplify GetNamespace and ListNamespace repository to populate provider information (with join) so we don't need to call two services (get namespace and get provider) in the service layer

[coveralls]

Pull Request Test Coverage Report for Build 3095620225

• 390 of 505 (77.23%) changed or added relevant lines in 27 files are covered.
• 28 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-4.8%) to 88.798%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/rule/service.go	20	21	95.24%
core/receiver/service.go	31	33	93.94%
core/provider/sync.go	0	3	0.0%
core/template/service.go	9	13	69.23%
plugins/providers/cortex/client.go	64	69	92.75%
plugins/receivers/httpreceiver/service.go	22	28	78.57%
plugins/receivers/pagerduty/service.go	22	28	78.57%
plugins/providers/cortex/subscription_service.go	0	9	0.0%
plugins/providers/cortex/service.go	26	37	70.27%
core/subscription/service.go	38	55	69.09%

Files with Coverage Reduction	New Missed Lines	%
core/receiver/service.go	2	96.52%
core/subscription/service.go	3	89.91%
core/receiver/receiver.go	10	0%
core/rule/service.go	13	81.33%

Totals
Change from base Build 3041921418:	-4.8%
Covered Lines:	2275
Relevant Lines:	2562

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💛 - Coveralls