Add SimpleHeaderMapper implementation [INT-1439]

Mark Fisher opened INT-1439 and commented

This implementation of HeaderMapper can be used for any case where the source/target type is simply a Map instance, e.g.:

public class SimpleHeaderMapper implements HeaderMapper<Map> {

   // something like this (and an inbound version)
   // see DefaultHttpHeaderMapper as an example
   public void setOutboundHeaderNames(String[] outboundHeaderNames) {

}

We would also need to support wildcard matching on the map keys, eg:

<something mapped-inbound-headers="foo, x-*"/>

For cases where we want to allow a Map of String keys and Expression values, we might want to add a new implementation, such as ExpressionEvaluatingHeaderMapper.

No further details from INT-1439

Start by reading the HeaderMapper contract and DefaultHttpHeaderMapper example mentioned in the issue. Define the scope for SimpleHeaderMapper, including inbound and outbound Map handling and wildcard key matching; completion would require a decided API and corresponding implementation and tests, but the issue names no repository files or tests.