DSPT-4967 provide alternate method for encoding and passing ext JSON

[drstevens]

Attempt to define an alternate method for Java implementation for passing ext and slot ext JSON without modifying original bid request content.

See README.md update

If sending via HTTP header, currently only available in Java implementation
1. retrieve protobuf generated message from Response, `Response.getExtProto`
2. Use this to generate a URI safe string using `ResponseUtil.encodedResponseMetadata`
3. Send this string as value for HTTP Header named `XAmazonTest: <encoded string>`

The goal is to avoid parsing the bid request string a second time just to insert ext JSON, only to serialize it back to bytes to send in HTTP Request body.

Unfortunately this meant making the ext JSON creation lazy and providing enough information to Response and Slot to generate both encodings.

[kchiang6997]

lgtm!