feat: Add mirror and mirror-token inputs for custom Python distribution sources

Users who need custom CPython builds (internal mirrors, GHES-hosted forks,
special build configurations, compliance builds, air-gapped runners) could not
previously point setup-python at anything other than actions/python-versions.

Adds two new inputs:
- `mirror`: base URL hosting versions-manifest.json and the Python
  distributions it references. Defaults to the existing
  https://raw.githubusercontent.com/actions/python-versions/main.
- `mirror-token`: optional token used to authenticate requests to the mirror.

If `mirror` is a raw.githubusercontent.com/{owner}/{repo}/{branch} URL, the
manifest is fetched via the GitHub REST API (authenticated rate limit applies);
otherwise the action falls back to a direct GET of {mirror}/versions-manifest.json.

Token interaction
-----------------

`token` is never forwarded to arbitrary hosts. Auth resolution is per-URL:

  1. if mirror-token is set, use mirror-token
  2. else if token is set AND the target host is github.com,
     *.github.com, or *.githubusercontent.com, use token
  3. else send no auth

Cases:

  Default (no inputs set)
    mirror = default raw.githubusercontent.com URL, mirror-token empty,
    token = github.token.
    → manifest API call and tarball downloads use `token`.
    Identical to prior behavior.

  Custom raw.githubusercontent.com mirror (e.g. personal fork)
    mirror-token empty, token = github.token.
    → manifest API call and tarball downloads use `token`
      (target hosts are GitHub-owned).

  Custom non-GitHub mirror, no mirror-token
    mirror-token empty, token = github.token.
    → manifest fetched via direct URL (no auth attached),
      tarball downloads use no auth.
    `token` is NOT forwarded to the custom host — this is the
    leak-prevention case.

  Custom non-GitHub mirror with mirror-token
    mirror-token set, token may be set.
    → manifest fetch and tarball downloads use `mirror-token`.

  Custom GitHub mirror with both tokens set
    mirror-token wins. Used for both the manifest API call and
    tarball downloads.

.github/workflows/test-python.yml

@@ -58,6 +58,26 @@ jobs:
       - name: Run simple code
         run: python -c 'import math; print(math.factorial(5))'
 
+  setup-versions-via-mirror-input:
+    name: 'Setup via explicit mirror input: ${{ matrix.os }}'
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: setup-python with explicit mirror
+        uses: ./
+        with:
+          python-version: 3.12
+          mirror: https://raw.githubusercontent.com/actions/python-versions/main
+
+      - name: Run simple code
+        run: python -c 'import sys; print(sys.version)'
+
   setup-versions-from-file:
     name: Setup ${{ matrix.python }} ${{ matrix.os }} version file
     runs-on: ${{ matrix.os }}