All instances of projectrekor are now renamed to SigStore

This includes:

* Import paths
* Tests
* Readme's

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

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

Co-authored-by: Dan Lorenc <dlorenc@google.com>

This allows us to skip sticking it onto each request context and retrieving it.

Signed-off-by: Dan Lorenc <dlorenc@google.com>

After #77, we have one global GRPC channel for the entire process. This
change causes each GRPC call to be made on the incoming HTTP request's
context such that if an HTTP client cancels prematurely, we will handle
the GRPC cleanup appropriately.

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>

simplify upload command to reuse common PKI package across client & server

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