dynamic modules: add abi.h to go mod vendor tree (#44395)

Commit Message: add abi.h to go mod vendor tree
Additional Description: go mod vendor is a common way in Golang to
import modules/packages. It works by downloading and storing the locked
version of a dependency in a vendor directory within the module.
Vendoring skips downloading and storing all directories that do not have
Go source files. This is a problem because the Go dynamic modules SDK
uses a CGO preamble with an import of this header file, but this header
is missing from the vendor download. This change adds a dummy Go file
that will make Go treat this directory as a valid Go package and thus
let it be imported so the header will exist properly.

Risk Level: none
Testing: n/a
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

---------

Signed-off-by: Adam Anderson <6754028+AdamEAnderson@users.noreply.github.com>

Author: AdamEAnderson

source/extensions/dynamic_modules/abi/doc.go

@@ -0,0 +1,7 @@
+// Package abi contains the C ABI header for Envoy dynamic modules.
+//
+// This file exists so that `go mod vendor` includes this directory.
+// Without a .go file, the Go toolchain skips this directory during
+// vendoring, which causes the abi.h header (needed by the Go SDK's
+// cgo include) to be missing from the vendor tree.
+package abi