chore: refactor Appearance to an interface callable by AGPL code (#11769)

The new Agent API needs an interface for ServiceBanners, so this PR creates it and refactors the AGPL and Enterprise code to achieve it.

Before we depended on the fact that the HTTP endpoint was missing to serve an empty ServiceBanner on AGPL deployments, but that won't work with dRPC, so we need a real interface to call.

coderd/coderd.go

@@ -17,10 +17,6 @@ import (
 	"sync/atomic"
 	"time"
 
-	"github.com/coder/coder/v2/coderd/prometheusmetrics"
-
-	agentproto "github.com/coder/coder/v2/agent/proto"
-
 	"github.com/andybalholm/brotli"
 	"github.com/go-chi/chi/v5"
 	"github.com/go-chi/chi/v5/middleware"
@@ -40,11 +36,12 @@ import (
 	"tailscale.com/util/singleflight"
 
 	"cdr.dev/slog"
+
+	agentproto "github.com/coder/coder/v2/agent/proto"
 	"github.com/coder/coder/v2/buildinfo"
 	"github.com/coder/coder/v2/cli/clibase"
-
-	// Used for swagger docs.
-	_ "github.com/coder/coder/v2/coderd/apidoc"
+	_ "github.com/coder/coder/v2/coderd/apidoc" // Used for swagger docs.
+	"github.com/coder/coder/v2/coderd/appearance"
 	"github.com/coder/coder/v2/coderd/audit"
 	"github.com/coder/coder/v2/coderd/awsidentity"
 	"github.com/coder/coder/v2/coderd/batchstats"
@@ -59,6 +56,7 @@ import (
 	"github.com/coder/coder/v2/coderd/httpapi"
 	"github.com/coder/coder/v2/coderd/httpmw"
 	"github.com/coder/coder/v2/coderd/metricscache"
+	"github.com/coder/coder/v2/coderd/prometheusmetrics"
 	"github.com/coder/coder/v2/coderd/provisionerdserver"
 	"github.com/coder/coder/v2/coderd/rbac"
 	"github.com/coder/coder/v2/coderd/schedule"
@@ -357,16 +355,6 @@ func New(options *Options) *API {
 		OIDC:   options.OIDCConfig,
 	}
 
-	staticHandler := site.New(&site.Options{
-		BinFS:         binFS,
-		BinHashes:     binHashes,
-		Database:      options.Database,
-		SiteFS:        site.FS(),
-		OAuth2Configs: oauthConfigs,
-		DocsURL:       options.DeploymentValues.DocsURL.String(),
-	})
-	staticHandler.Experiments.Store(&experiments)
-
 	ctx, cancel := context.WithCancel(context.Background())
 	r := chi.NewRouter()
 
@@ -383,7 +371,6 @@ func New(options *Options) *API {
 		ID:          uuid.New(),
 		Options:     options,
 		RootHandler: r,
-		SiteHandler: staticHandler,
 		HTTPAuth: &HTTPAuthorizer{
 			Authorizer: options.Authorizer,
 			Logger:     options.Logger,
@@ -412,6 +399,19 @@ func New(options *Options) *API {
 			options.Database,
 			options.Pubsub),
 	}
+
+	api.AppearanceFetcher.Store(&appearance.DefaultFetcher)
+	api.SiteHandler = site.New(&site.Options{
+		BinFS:             binFS,
+		BinHashes:         binHashes,
+		Database:          options.Database,
+		SiteFS:            site.FS(),
+		OAuth2Configs:     oauthConfigs,
+		DocsURL:           options.DeploymentValues.DocsURL.String(),
+		AppearanceFetcher: &api.AppearanceFetcher,
+	})
+	api.SiteHandler.Experiments.Store(&experiments)
+
 	if options.UpdateCheckOptions != nil {
 		api.updateChecker = updatecheck.New(
 			options.Database,
@@ -1089,6 +1089,7 @@ type API struct {
 	TailnetCoordinator                atomic.Pointer[tailnet.Coordinator]
 	TailnetClientService              *tailnet.ClientService
 	QuotaCommitter                    atomic.Pointer[proto.QuotaCommitter]
+	AppearanceFetcher                 atomic.Pointer[appearance.Fetcher]
 	// WorkspaceProxyHostsFn returns the hosts of healthy workspace proxies
 	// for header reasons.
 	WorkspaceProxyHostsFn atomic.Pointer[func() []string]