This sets the User-Agent header to match the format used in cosign of

rekor-cli/vX.Y.Z (OS) (ARCH)

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

Signed-off-by: Scott Nichols <n3wscott@chainguard.dev>

* rekor-server: Implement /api/v1/version

This implements a version endpoint for rekor. This helps figure out the
version the server is currently running. It could later be used to
implement version compatibility with CLI utilities.

Example:
   λ rekor main» curl -s localhost:3000/api/v1/version | jq
   {
     "builddate": "'2021-12-27T13:20:32Z'",
     "commit": "12d1a47c",
     "treestate": "dirty",
     "version": "v0.4.0-15-g12d1a47-dirty"
   }

This removes some duplication of the build flags and inserts them into
/pkg/api which is then reused across the utilities.

Signed-off-by: Morten Linderud <morten@linderud.pw>

Signed-off-by: Morten Linderud <morten@linderud.pw>

* Generated files

Signed-off-by: Morten Linderud <morten@linderud.pw>

Signed-off-by: Carlos Panato <ctadeu@gmail.com>

* 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>