Add support for Buildkite OIDC tokens

Add minimal support required to generate attestations for rubygems
trusted publisher support in a Buildkite job

1. Teach sigstore-cli gem how to detect when it's running in a Buildkite
   job, and how to request a Buildkite OIDC token automatically
2. Teach sigstore gem how to parse an Buildkite OIDC token and generate
   the identity that fulcio uses for the x509 common name in
   certificates for Buildkite OIDC tokens

With these changes, the following command works in a Buildkite job:

    bundle exec sigstore-cli sign <somefile> --bundle bundle.sigstore.json

I've tested this using the pipeline defined in a gist:
https://gist.github.com/yob/512f079e0586d49d125547ad548c08b5

The pipeline is private [1], however an example rekor log for a bundle
created using this branch is at [2]

[1] https://buildkite.com/yob-opensource/sigstore-ruby-test/
[2] https://search.sigstore.dev/?logIndex=176164097

Signed-off-by: James Healy <james@yob.id.au>

Author: yob

cli/lib/sigstore/cli/id_token.rb

@@ -9,9 +9,9 @@ class AmbientCredentialError < Sigstore::Error
 
     def self.detect_credential
       [
-        GitHub
+        GitHub,
+        Buildkite
         # detect_gcp,
-        # detect_buildkite,
         # detect_gitlab,
         # detect_circleci
       ].each do |detector|
@@ -85,5 +85,34 @@ def call
         end
       end
     end
+
+    class Buildkite < IdToken
+
+      def call
+        logger.debug { "looking for OIDC credentials" }
+        unless ENV["BUILDKITE"]
+          logger.debug { "environment doesn't look like Buildkite; giving up" }
+          return
+        end
+
+        raise AmbientCredentialError, "buildkite-agent executable not found" unless buildkite_agent_found?
+
+        request_token
+      end
+
+      private
+
+      def buildkite_agent_found?
+        `which buildkite-agent`
+        $?.success?
+      end
+
+      def request_token
+        token = `buildkite-agent oidc request-token --audience 'sigstore'`
+        raise AmbientCredentialError, "error requesting Buildkite OIDC token" unless $?.success?
+
+        token
+      end
+    end
   end
 end

lib/sigstore/oidc.rb

@@ -20,7 +20,8 @@ module OIDC
       "https://accounts.google.com" => "email",
       "https://oauth2.sigstore.dev/auth" => "email",
       "https://oauth2.sigstage.dev/auth" => "email",
-      "https://token.actions.githubusercontent.com" => "job_workflow_ref"
+      "https://token.actions.githubusercontent.com" => "job_workflow_ref",
+      "https://agent.buildkite.com" => "pipeline_slug"
     }.freeze
     private_constant :KNOWN_OIDC_ISSUERS
 
@@ -47,8 +48,19 @@ def initialize(raw_token)
           end
 
           @identity = @unverified_claims[identity_claim]
-          # https://github.com/sigstore/fulcio/blob/8311f93c01ea5b068a86d37c4bb51573289bfd69/pkg/identity/github/principal.go#L92
-          @identity = "https://github.com/#{@identity}" if issuer == "https://token.actions.githubusercontent.com"
+          if issuer == "https://token.actions.githubusercontent.com"
+            # https://github.com/sigstore/fulcio/blob/8311f93c01ea5b068a86d37c4bb51573289bfd69/pkg/identity/github/principal.go#L92
+            @identity = "https://github.com/#{@identity}"
+          elsif issuer == "https://agent.buildkite.com"
+            # https://github.com/sigstore/fulcio/blob/ec8a1d7a96125a1a624b9e69df892f987bebc41c/config/identity/config.yaml#L241
+            org_slug = @unverified_claims["organization_slug"]
+            if org_slug.nil?
+              raise Error::InvalidIdentityToken,
+                    "identity token is missing required claim: organization_slug"
+            end
+
+            @identity = "https://buildkite.com/#{org_slug}/#{@identity}"
+          end
         else
           @identity = @unverified_claims["sub"]
         end