refactor(extensions)!: refactor the extensions URLs and errors (#1636)

BREAKING CHANGE: The functionality provided by the mgmt endpoint has beed redesigned - see details below
BREAKING CHANGE: The API keys endpoint has been moved -  see details below
BREAKING CHANGE: The mgmt extension config has been removed - endpoint is now enabled by having both the search and the ui extensions enabled
BREAKING CHANGE: The API keys configuration has been moved from extensions to http>auth>apikey

mgmt and imagetrust extensions:
- separate the _zot/ext/mgmt into 3 separate endpoints: _zot/ext/auth, _zot/ext/notation, _zot/ext/cosign
- signature verification logic is in a separate `imagetrust` extension
- better hanling or errors in case of signature uploads: logging and error codes (more 400 and less 500 errors)
- add authz on signature uploads (and add a new middleware in common for this purpose)
- remove the mgmt extension configuration - it is now enabled if the UI and the search extensions are enabled

userprefs estension:
- userprefs are enabled if both search and ui extensions are enabled (as opposed to just search)

apikey extension is removed and logic moved into the api folder
- Move apikeys code out of pkg/extensions and into pkg/api
- Remove apikey configuration options from the extensions configuration and move it inside the http auth section
- remove the build label apikeys

other changes:
- move most of the logic adding handlers to the extensions endpoints out of routes.go and into the extensions files.
- add warnings in case the users are still using configurations with the obsolete settings for mgmt and api keys
- add a new function in the extension package which could be a single point of starting backgroud tasks for all extensions
- more clear methods for verifying specific extensions are enabled
- fix http methods paired with the UI handlers
- rebuild swagger docs

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

Makefile

@@ -32,8 +32,8 @@ TESTDATA := $(TOP_LEVEL)/test/data
 OS ?= linux
 ARCH ?= amd64
 BENCH_OUTPUT ?= stdout
-EXTENSIONS ?= sync,search,scrub,metrics,lint,ui,mgmt,userprefs,apikey
-UI_DEPENDENCIES := search,mgmt,userprefs,apikey
+EXTENSIONS ?= sync,search,scrub,metrics,lint,ui,mgmt,userprefs,imagetrust
+UI_DEPENDENCIES := search,mgmt,userprefs
 # freebsd/arm64 not supported for pie builds
 BUILDMODE_FLAGS := -buildmode=pie
 ifeq ($(OS),freebsd)