Add simple facility for an extendable shared memory area (#11929)

You still need to provide a max size up-front, but memory is only
allocated for the portion that is in use.

The module is currently unused, but will be used by the new compute
communicator project, in the neon Postgres extension. See
https://github.com/neondatabase/neon/issues/11729

---------

Co-authored-by: Erik Grinaker <erik@neon.tech>

Cargo.lock

@@ -3794,6 +3794,16 @@ version = "0.8.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e5ce46fe64a9d73be07dcbe690a38ce1b293be448fd8ce1e6c1b8062c9f72c6a"
 
+[[package]]
+name = "neon-shmem"
+version = "0.1.0"
+dependencies = [
+ "nix 0.30.1",
+ "tempfile",
+ "thiserror 1.0.69",
+ "workspace_hack",
+]
+
 [[package]]
 name = "never-say-never"
 version = "6.6.666"
@@ -8482,6 +8492,7 @@ dependencies = [
  "log",
  "memchr",
  "nix 0.26.4",
+ "nix 0.30.1",
  "nom",
  "num",
  "num-bigint",