Implement /api/v1/version for rekor-server (#569)

* 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>