Consider using code generation (ex: easyjson) for JSON models

[Luke Young]

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Description

Currently go-audit uses the stdlib encoding/json class to marshal the AuditMessageGroup and AuditMessage structs into a []byte. Internally the encoding/json package uses reflection so it can marshal arbitrary objects.

Because go-audit only uses two static structures there would likely be a significant performance improvement using a code generation library like easyjson to serialize the structures into bytes.

You can find various benchmarks on JSON serialization performance across code generation packages vs stdlib but I am partial to go_serialization_benchmarks:

benchmark	iter	time/iter	bytes/op	allocs/op	tt.sec	tt.kb	ns/alloc
BenchmarkJsonMarshal-8	1000000	1585 ns/op	304	4	1.58	30400	396.25
BenchmarkEasyJsonMarshal-8	1000000	1125 ns/op	784	5	1.12	78400	225.00

This comes at the cost of an additional dependency (primarily when the models change only)