WIP: Add custom jlink/jshell example

[msgilligan]

This is WIP, but gradle secp-examples-jshell:jlink should build an image with a jshell that contains
libsecp256k1 and supports the following (for example)

import org.bitcoinj.secp.api.*
var secp = Secp256k1.get()

This will load the FFM implementation.

Issues:

• The gradle secp-examples-jshell:jlink target depends on the LIBSECP_DIR which is currently only set up in the Nix devShell.
• I haven't tried building under CI or on platforms other than macOS
• I'm not sure if the copyJdkSource task is working properly
• I haven't figured out how to get secp-api source configured in jshell (if possible)

[msgilligan]

Note that gradle build jlink does not do a full, proper build. It is sometimes necessary to do gradle clean build jlink 🙁

[msgilligan]

There really is no reason to build a custom jlink image just for this purpose. All that is needed is a launcher script and/or some documentation on how to do it in the user guide.