ssh: add generic Match support for matchBlocks (#2992)

* ssh: add generic Match support for matchBlocks

Introduce conservative support for actual `Match`
blocks in ssh config.

"Conservative" means this PR doesn'tt try to process
the `match` expression and simply uses it as a string
provided by the user.

If set, `match` has precedence over `host` meaning
if both are set, `match` is used and `host` is ignored.

* Add news entry

tests/modules/programs/ssh/default.nix

@@ -2,6 +2,7 @@
   ssh-defaults = ./default-config.nix;
   ssh-includes = ./includes.nix;
   ssh-match-blocks = ./match-blocks-attrs.nix;
+  ssh-match-blocks-match-and-hosts = ./match-blocks-match-and-hosts.nix;
 
   ssh-forwards-dynamic-valid-bind-no-asserts =
     ./forwards-dynamic-valid-bind-no-asserts.nix;