All instances of projectrekor are now renamed to SigStore

This includes:

* Import paths
* Tests
* Readme's

Signed-off-by: Luke Hinds <lhinds@redhat.com>

Clear up the stuff we no longer use and add some basic files
for test purposes (integration tests / local developement)

Implement upload command

Accept flags artifact-url,public-key, signature and validate GPG formatting and verify file signature