Add deb virtual repositories (#138)

Virtual repos merge helm, pypi, rpm, npm, gems, ansible, goproxy and puppet but not deb, so several apt repos (e.g. github_deb remotes plus a local deb) cannot be served as one apt source.

- merge member Packages for flat (`./`) and `dists/<suite>` layouts; member order wins duplicate (Package, Version, Architecture)
- generate an unsigned Release with Acquire-By-Hash, serving indexes by name and by SHA256 from both retained generations
- prefix Filename with the member name and 302 downloads to the member route
- cache merges per suite on the shared virtual cache, capped at 32 suites per virtual (LRU)
- add docs/deb/virtual.md, unit tests and dockerised apt e2e for flat and suite layouts

Reviewed-on: #138
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

docs/deb/virtual.md

@@ -0,0 +1,32 @@
+# `deb` virtual
+
+A deb virtual serves one apt source over local and remote deb members at
+`/api/v1/virtual/<name>/`, in either the flat (`./`) or `dists/<suite>` layout.
+
+- Packages indexes are merged; on a duplicate (Package, Version, Architecture)
+  the earlier member wins.
+- The Release is generated unsigned with `Acquire-By-Hash: yes`; indexes are
+  served by name and by SHA256 from the current and previous merge.
+- `Filename` points at the virtual (`<member>/<path>`), which 302s to the
+  owning member.
+- All members must share one layout; mixing flat and suite members is
+  unsupported (suite requests fail with 502).
+- A member error fails the request with 502. Merges are cached for 60s per
+  suite, up to 32 suites per virtual.
+
+```hcl
+resource "artifactapi_virtual" "deb" {
+  name         = "deb"
+  package_type = "deb"
+  members      = [artifactapi_local_deb.internal.name, artifactapi_remote_github_deb.tool.name]
+}
+```
+
+```
+# /etc/apt/sources.list.d/artifactapi.list (flat)
+deb [trusted=yes] https://artifactapi.example.com/api/v1/virtual/deb/ ./
+# suite layout
+deb [trusted=yes] https://artifactapi.example.com/api/v1/virtual/debian/ trixie main
+```
+
+Upload to the local member directly; the virtual is read-only.