[#498] middleware: Add spans to detail the trace

Spans are added only to the following middleware:
* PolicyCheck
* Auth
* FrostfsIDValidation

This is done this way because these middleware are basic and
they interact with frostfs-storage.

Also, an explicit context has been added to many functions
so that the middleware spans do not include all subsequent spans.

Signed-off-by: Roman Loginov <r.loginov@yadro.com>

api/router_mock_test.go

@@ -45,7 +45,7 @@ type centerMock struct {
 	key          *keys.PrivateKey
 }
 
-func (c *centerMock) Authenticate(*http.Request) (*middleware.Box, error) {
+func (c *centerMock) Authenticate(context.Context, *http.Request) (*middleware.Box, error) {
 	if c.noAuthHeader {
 		return nil, middleware.ErrNoAuthorizationHeader
 	}