WIP - Decimal support

[simplesteph]

trafficstars

A big feature that was missing from the Rest proxy is the ability to send decimal fields. As it stands it requires the user to figure out a string to send, which is super risky and quite a complicated process.

Instead, my wish is for the rest proxy to take {"foo": 123.45 } as an input, and do the conversion to avro correctly on its own. On read of decimal fields, it should do the opposite operation and return {"foo": 123.45 }

I have added test cases that demonstrate all of this.

I look forward to feedback. The one thing I'm not happy with is the use of "ISO-8859-1". If you see a simpler solution please do let me know.

The accuracy of this has been tested on a wide range of decimal numbers, and the deserialisation code from byte to BigDecimal has been directly sourced from the master version of avro at: https://github.com/apache/avro/blob/33d495840c896b693b7f37b5ec786ac1acacd3b4/lang/java/avro/src/main/java/org/apache/avro/Conversions.java#L79

Needless to say, an update to the avro library would greatly simplify this PR

[ConfluentJenkins]

Can one of the admins verify this patch?

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