Switch shims from vendor to external cell

Summary: With this change, the shims repo will be disconnected from the build process of each project, and we will use buck2's native external cells feature instead.

Reviewed By: bigfootjon

Differential Revision: D64582392

fbshipit-source-id: b23894b5bf4dd8dfb20aecda29be341aa7c68f8f

.buckconfig.d/external_cells/facebook/buck2-shims-meta/external_cell.buckconfig

@@ -0,0 +1,9 @@
+[cells]
+gh_facebook_buck2_shims_meta = gh_facebook_buck2_shims_meta
+
+[external_cells]
+gh_facebook_buck2_shims_meta = git
+
+[external_cell_gh_facebook_buck2_shims_meta]
+git_origin = https://github.com/facebook/buck2-shims-meta.git
+commit_hash = 971b2cb76e43b31983d868c6ccb9ee51c441f6e8