POC Trunk binary distribution format

[theory]

trunk.mk copies a bunch of the install target from PGXS and modifies it to install into a organized according to the proposed format. The adds the make variables DISTVERSION, LICENSE, and LANGUAGE, but otherwise depends on variables defined by PGXS. It uses jq to build trunk.json, shasum to build the digests file, and tar to create the *.trunk artifact as a tarball.

install_trunk is a simple shell script to demonstrate unpacking, validating, and installing a trunk file. It too relies on jq, shasum, and tar, and uses rsync to do the actual installation. It also does basic trunk version, platform, and Postgres version validation.