Support object comparison for SelfDescribingJson

[stephen-murby]

Unit testing is made difficult for consumers of the library without being able to compare like library types.

We should implement and override the equals and hashCode methods for SelfDescribingJson & SelfDescribing

[stephen-murby]

An example of the unit tests is as such

val actual = SnowplowCustomContexts.Builder.from(serverSideEvent)
val expected = listOf( SelfDescribingJson("iglu:uk.co.autotrader/custom_context/jsonschema/1-0-0", mapOf("field" to "value")) )

assertThat(actual.toString()).isEqualTo(expected.toString()) // Currently the only way to compare
assertThat(actual.).isEqualTo(expected) // Would rather do this

[stephen-murby]

Even if we only go as far as comparing this.toString() to that.toString(); it would stop the user having to do this themselves over and over again.