JsonJacksonApprovals should accept instance of ObjectMapper

[olsavmic]

The org.approvaltests.JsonJacksonApprovals class should accept ObjectMapper as an optional parameter, similarly to the org.approvaltests.JsonApprovals accepting a Function1<GsonBuilder, GsonBuilder> gsonBuilder.

The main motivation is to have consistent serialization with the rest of the application, allowing to perform pretty-printing without changing the semantics of the output (typically NULL values present vs missing).

I don't think there is any reason to modify the mapper during verification, therefore I find it safe to pass the actual instance instead of some ObjectMapper factory/builder.

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Thank you for the tooling! I really appreciate such a zero-config snapshotting functionality :)

[olsavmic]

Since Jackson supports ordering of properties and map entries out of the box (ORDER_MAP_ENTRIES_BY_KEYS, SORT_PROPERTIES_ALPHABETICALLY options), allowing to pass an instance of ObjectMapper will make the JsonJacksonApprovals implementation on par with JsonApprovals in terms of features.