Add version subcommands to rekor-cli & rekor-server (#304)

* Add version subcommands to rekor-cli & rekor-server

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>

* Add version info to docker build & print at startup

This ensures the build ldflags are applied as part of the docker build
process (for docker-compose and ko).

This also prints the version information of the running server to the
logs upon server startup.

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>