support VURL backend

Not yet implemented is recording hashes on download from web and
verifying hashes.

addurl --verifiable option added with -V short option because I
expect a lot of people will want to use this.

It seems likely that --verifiable will become the default eventually,
and possibly rather soon. While old git-annex versions don't support
VURL, that doesn't prevent using them with keys that use VURL. Of
course, they won't verify the content on transfer, and fsck will warn
that it doesn't know about VURL. So there's not much problem with
starting to use VURL even when interoperating with old versions.

Sponsored-by: Joshua Antonishen on Patreon

Types/Key.hs

@@ -1,6 +1,6 @@
 {- git-annex Key data type
  -
- - Copyright 2011-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2024 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -218,6 +218,7 @@ data KeyVariety
 	| MD5Key HasExt
 	| WORMKey
 	| URLKey
+	| VURLKey
 	-- A key that is handled by some external backend.
 	| ExternalKey S.ByteString HasExt
  	-- Some repositories may contain keys of other varieties,
@@ -251,6 +252,7 @@ hasExt (SHA1Key (HasExt b)) = b
 hasExt (MD5Key (HasExt b)) = b
 hasExt WORMKey = False
 hasExt URLKey = False
+hasExt VURLKey = False
 hasExt (ExternalKey _ (HasExt b)) = b
 hasExt (OtherKey s) = (snd <$> S8.unsnoc s) == Just 'E'
 
@@ -279,6 +281,7 @@ formatKeyVariety v = case v of
 	MD5Key e -> adde e "MD5"
 	WORMKey -> "WORM"
 	URLKey -> "URL"
+	VURLKey -> "VURL"
 	ExternalKey s e -> adde e ("X" <> s)
 	OtherKey s -> s
   where
@@ -343,6 +346,7 @@ parseKeyVariety "MD5"          = MD5Key (HasExt False)
 parseKeyVariety "MD5E"         = MD5Key (HasExt True)
 parseKeyVariety "WORM"         = WORMKey
 parseKeyVariety "URL"          = URLKey
+parseKeyVariety "VURL"         = VURLKey
 parseKeyVariety b
 	| "X" `S.isPrefixOf` b = 
 		let b' = S.tail b