third_party: add libpg_query and sqlc

This adds sqlc, a SQL query code generator for Go (and other languages).
It in turn requires pganalyze's libpg_query, which is a C library for
parsing PostgreSQL queries.

To test:

   $ bazel build @com_github_kyleconroy_sqlc//cmd/sqlc

In the future this will be used by Bazel rules to generate sources at
build time.

Change-Id: I369c9ab503e8ce6952fd3f73c233dd3d59922358
Reviewed-on: https://review.monogon.dev/c/monogon/+/882
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/WORKSPACE b/WORKSPACE
index da64ef6..42bd34d 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -238,6 +238,12 @@
     version = "3542da2442d8b29661b47c42ad7e5fa9bc8562ec",
 )
 
+load("//third_party/libpg_query:external.bzl", "libpg_query_external")
+libpg_query_external(
+    name = "libpg_query",
+    version = "13-2.1.2",
+)
+
 register_toolchains("//:host_python")
 
 # same for gvisor/rules_docker.