feat(provider): implement per-alert limits

[siavashs]

Add new limit package with bucket

Add a new limit package with generic bucket implementation.
This can be used for example to limit the number of alerts in memory.

Benchmarks:

goos: darwin
goarch: arm64
pkg: github.com/prometheus/alertmanager/limit
cpu: Apple M3 Pro
BenchmarkBucketUpsert/EmptyBucket-12  	 8816954	       122.4 ns/op	      56 B/op	       2 allocs/op
BenchmarkBucketUpsert/AddToFullBucketWithExpiredItems-12         	 9861010	       123.0 ns/op	      56 B/op	       2 allocs/op
BenchmarkBucketUpsert/AddToFullBucketWithActiveItems-12          	 8343778	       143.6 ns/op	      56 B/op	       2 allocs/op
BenchmarkBucketUpsert/UpdateExistingItem-12                     	10107787	       118.9 ns/op	      56 B/op	       2 allocs/op
BenchmarkBucketUpsert/MixedWorkload-12                           	 9436174	       126.0 ns/op	      56 B/op	       2 allocs/op
BenchmarkBucketUpsertScaling/BucketSize_10-12                    	10255278	       115.4 ns/op	      56 B/op	       2 allocs/op
BenchmarkBucketUpsertScaling/BucketSize_50-12                    	10166518	       117.1 ns/op	      56 B/op	       2 allocs/op
BenchmarkBucketUpsertScaling/BucketSize_100-12                   	10457394	       115.0 ns/op	      56 B/op	       2 allocs/op
BenchmarkBucketUpsertScaling/BucketSize_500-12                   	 9644079	       115.2 ns/op	      56 B/op	       2 allocs/op
BenchmarkBucketUpsertScaling/BucketSize_1000-12                  	10426184	       116.6 ns/op	      56 B/op	       2 allocs/op
BenchmarkBucketUpsertConcurrent-12                               	 5796210	       216.3 ns/op	     406 B/op	       5 allocs/op
PASS
ok  	github.com/prometheus/alertmanager/limit	15.497s

Implement per-alert limits

Use the new limit module to add optional per alert-name limits.

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Signed-off-by: Siavash Safi [email protected]