add cabal.project file for GHC 9.0.1 experiments

usage:
cabal build messaging-client-terminal --project-file=ghc9.cabal.project

Stack seems to break with GHC 9.0.1:
commercialhaskell/stack#5486

With Cabal everything except the GTK client builds, but for that we need
a Hackage release of gi-haskell-0.25.0:
haskell-gi/haskell-gi#336

ghc9.cabal.project

@@ -0,0 +1,14 @@
+packages: */*.cabal
+
+constraints:
+  base >= 4.15,
+  -- not yet available on Hackage :(
+  haskell-gi >= 0.25
+
+
+allow-newer:
+  cryptohash-sha1:base,
+  cryptohash-md5:base,
+  entropy:Cabal,
+  gi-gtk-declarative:haskell-gi,
+  gi-gtk-declarative-app-simple:haskell-gi