sbt-iglu-push: add

[chuwy]

Just updated a schema in one of embedded registries and realized how easy it can get out of sync with main Iglu registry.

Illustrative scenario: we have some schema-ed Foo(enum: Bar | Baz) entity in a spark job that needs to be:

1. Parsed from JSON
2. Validated by Iglu client
3. Decoded into Foo(Bar) or Foo(Baz) class

Decoder knows that Bar and Baz are acceptable values. However once we add Qux to list of acceptable values - schema will get out of sync.

With sbt-iglu-push we can configure an SBT task to derive schema from decoder and push it to Iglu Server during CI/CD.

Kind of reverse of #88 (which is Schema -> Class definition, while we have Class definition -> Schema, which is much more approachable)