fix: various lint issues (#2571)

Originally found by @jkroepke in: https://github.com/project-zot/zot/actions/runs/10056774230/job/27796324933?pr=2556
And https://github.com/project-zot/zot/pull/2556 in general

This commit covers:
- (canonicalheader) capitalization of Docker-Content-Digest header
- (protogetter) the proto getters were not used, they check for nil pointers, we should switch to using them
- (zerologlint) fix the false positive

Signed-off-by: Andrei Aaron <aaaron@luxoft.com>

pkg/api/session.go

@@ -89,7 +89,7 @@ func SessionLogger(ctlr *Controller) mux.MiddlewareFunc {
 			clientIP := request.RemoteAddr
 			method := request.Method
 			headers := map[string][]string{}
-			log := logger.Info()
+			log := logger.Info() //nolint: zerologlint // false positive, the Msg call is below
 			for key, value := range request.Header {
 				if key == "Authorization" { // anonymize from logs
 					s := strings.SplitN(value[0], " ", 2) //nolint:gomnd