Preview of cabal-install based builds (See hasura#3280)

The idea here is to start the process of moving away from stack
(potentially), but making this move official if we decie to, will
require more coordination (moving CI, dev.sh and docs all at once).

I've added a "fast" variation of cabal.project.local mostly as a
demonstration. I haven't found a "prof" variant that really works for
me. All this is pretty open to change, but it would be nice to have some
local cabal.project files that encode good/useful stuff.

The freeze file should be version-exact to our stackage LTS snapshot.

server/.gitignore

@@ -17,6 +17,8 @@ Pipfile.lock
 .cabal-sandbox/
 cabal.sandbox.config
 cabal.config
+# Configurable for local dev/experimentation:
+cabal.project.local
 *.prof*
 *.aux
 *.hp

server/cabal.project

@@ -0,0 +1,28 @@
+---- PREVIEW OF CABAL-INSTALL WORKFLOW
+----   Try: 
+----     $ cabal new-build
+----   See: 
+----     https://github.com/hasura/graphql-engine/issues/3280 
+----     https://www.haskell.org/cabal/users-guide/nix-local-build-overview.html 
+packages: .
+
+package *
+  optimization: 2
+
+package graphql-engine
+  ghc-options: -j
+
+source-repository-package
+  type: git
+  location: https://github.com/hasura/pg-client-hs.git
+  tag: bd21e66d8197af381a6c0b493e22d611ed1fa386 
+
+source-repository-package
+  type: git
+  location: https://github.com/hasura/graphql-parser-hs.git
+  tag: 2e8adedbb426d487df77bde244b7fe3cbd40a255
+
+source-repository-package
+  type: git
+  location: https://github.com/hasura/ci-info-hs.git
+  tag: 6af5a68450347a02295a9cd050d05a8b2f5c06ab