* Add support for ed25519/signify/minisign keys and signatures

Signed-off-by: Bob Callaway <bcallawa@redhat.com>

QueuedLeafResponse holds the actual insertion response code, so check that instead of just the base GRPC status code.

also fixes middleware ordering issue

Signed-off-by: Bob Callaway <bcallawa@redhat.com>

- adds trillian API object to context
- improves request error logging pre and post validation
- use consistent request context throughout all GRPC calls
- improve validation of incoming UUID values
- disable swagger UI endpoint
- only print cacheable headers if response code is HTTP 2XX
- use GetLeavesByRange instead of deprecated GetLeavesByIndex API

Signed-off-by: Bob Callaway <bcallawa@redhat.com>

* add types unit tests

* explicitly ignore error from ValidateLeaf

* improve PGP unit test coverage (#58)

* update & improve pgp unit tests

* add types unit tests

* explicitly ignore error from ValidateLeaf

* use files instead of embedded strings

* update & improve pgp unit tests

Some changes:
- import names (app -> api, logging -> log)
- Commands package (cmd -> app)

This also breaks the go module dependency on rekor-server.

This is the first step in code reorganization. This follows the patterns
from kubernetes/kubernetes - each binary gets a directory under cmd/.
The main.go file for each binary lives there.
The commands live under cmd//app.

Implement upload command

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

Seems to work for Node right now. I haven't tested on linux.