feat: add silencing notification

[mabdh]

proto PR is here raystack/proton#237

• add silences feature
• add some new tables to support silences feature
• refactor notification service
• refactor post alerts api
• refactor post notification receiver api

[coveralls]

Pull Request Test Coverage Report for Build 4060590481

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• 614 of 790 (77.72%) changed or added relevant lines in 26 files are covered.
• 12 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.1%) to 77.1%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/notification/notification.go	21	22	95.45%
internal/api/v1beta1/alert.go	12	13	92.31%
internal/store/postgres/subscription.go	3	6	50.0%
core/notification/dispatch_subscriber_service.go	107	111	96.4%
core/silence/silence.go	37	41	90.24%
internal/api/v1beta1/silence.go	62	66	93.94%
internal/store/postgres/alerts.go	9	13	69.23%
core/notification/service.go	46	51	90.2%
plugins/providers/cortex/service.go	0	5	0.0%
core/silence/type.go	0	6	0.0%

Files with Coverage Reduction	New Missed Lines	%
core/notification/notification.go	1	91.67%
plugins/queues/postgresq/queue.go	2	72.62%
core/notification/service.go	3	82.29%
plugins/queues/inmemory/queue.go	6	49.12%

Totals
Change from base Build 3929358348:	-0.1%
Covered Lines:	3360
Relevant Lines:	4358

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💛 - Coveralls