Add gems virtual repositories

Merge the compact index across gems members and redirect downloads to the owning member.

docs/gems/virtual.md

@@ -0,0 +1,28 @@
+# `gems` virtual
+
+A gems virtual serves one RubyGems source over local and remote gems members:
+`gem install` and `bundle install` work against `/api/v1/virtual/<name>/`.
+
+- `info/<gem>` is the union of member lines; on a duplicate version-platform
+  the earlier member wins. A gem only one member has is served verbatim.
+- `versions` regenerates the line (and md5) of every gem several members list
+  from its merged `info`; other gems' lines pass through. `names` is the union.
+- `gems/*.gem` and `quick/Marshal.4.8/*.gemspec.rz` 302 to the member whose
+  `info` line won.
+- A member error fails the request with 502; a member without the gem is
+  skipped. Merges are cached for 60s.
+
+```hcl
+resource "artifactapi_virtual" "gems" {
+  name         = "gems"
+  package_type = "gems"
+  members      = [artifactapi_local_gems.internal.name, artifactapi_remote_gems.rubygems.name]
+}
+```
+
+```ruby
+# Gemfile
+source "https://artifactapi.example.com/api/v1/virtual/gems/"
+```
+
+Push to the local member directly; the virtual is read-only.