A kamelet should define which headers are supported as input and output

We should start to markup/specify which headers a sink and source kamelet can support as input and output.
For example the ftp sink kamelet should specify that the kamelet outputs `ce-file` as the filename of the FTP file.

The underlying FTP component also provides a set of headers
https://camel.apache.org/components/next/ftp-component.html#_message_headers

From Camel 3.17 then every component now list those in their documentation and metadata json file.

We need to consider whether we want Kamelets to expose these headers too, or we make a rule into kamelets that they output headers that the kamelet has declared.

And we also need to add into the kamelet spec how headers should be declared (ala options today).

Start by reviewing the FTP sink Kamelet and the linked FTP component message-header documentation. Define how input and output headers should be declared in the Kamelet spec, including whether underlying component headers are exposed, and what rule determines the supported header set.

Written by the indexing model from the issue text.