create a toHeader SMT

[valdar]

That SMT should take fields from a configured record value (supporting struct + schema or map cases) and copy the value in a configure header like:

transforms=toHeader
transforms.toHeader.type=org.apache.camel.kafkaconnect.transforms.ToHeader
transforms.toHeaderader.fields=to.address,cc.addresses
transforms.toHeaderr.headers=CamelHeader.to,CamelHeader.cc

this can be used as a base for extracting fields https://github.com/apache/kafka/blob/trunk/connect/transforms/src/main/java/org/apache/kafka/connect/transforms/ExtractField.java

[valdar]

should help with #644

[valdar]

@luigidemasi fancy having a look?