Enable vcpkg cache via vcpkg-cache-action (#584)

- a new GitHub Action is introduced to provide vcpkg binary caching via a custom
  HTTP server that proxies to GitHub Actions cache
- the result is granular caching of vcpkg artifacts, dramatically reducing build
  times
- the action is a typescript shim that invokes a Python script (this is so we
  can register a post step)
- the shim is "built" via "npm build" and the resulting JavaScript is checked
  into source control (best practice for GitHub Actions)
- the Python script is launched via pipx and uv (as uv itself is not installed
  on the GitHub Actions runners)
- uv is invoked with the --locked option to ensure consistent dependency
  resolution
- the bulk of the action is written in Python for cross-platform compatibility
  and fast startup times without requiring a separate compilation step
- the Python script is responsible for both the main and post steps of the
  action and sets the VCPKG_BINARY_SOURCES environment variable accordingly

Author: jgiannuzzi

.github/actions/vcpkg-cache-action/.gitignore

@@ -0,0 +1,4 @@
+lib/
+node_modules/
+__pycache__/
+.venv/

.github/actions/vcpkg-cache-action/.python-version

@@ -0,0 +1 @@
+3.12

.github/actions/vcpkg-cache-action/action.yml

@@ -0,0 +1,7 @@
+name: vcpkg cache action
+description: GitHub Action to provide vcpkg binary caching via a custom HTTP server that proxies to GitHub Actions cache
+
+runs:
+  using: node24
+  main: dist/index.js
+  post: dist/index.js