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.

enterprise/coderd/coderd.go

@@ -14,6 +14,8 @@ import (
 	"sync"
 	"time"
 
+	"github.com/coder/coder/v2/coderd/appearance"
+
 	"golang.org/x/xerrors"
 	"tailscale.com/tailcfg"
 
@@ -118,7 +120,6 @@ func New(ctx context.Context, options *Options) (_ *API, err error) {
 	}
 	api.AGPL.Options.SetUserGroups = api.setUserGroups
 	api.AGPL.Options.SetUserSiteRoles = api.setUserSiteRoles
-	api.AGPL.SiteHandler.AppearanceFetcher = api.fetchAppearanceConfig
 	api.AGPL.SiteHandler.RegionsFetcher = func(ctx context.Context) (any, error) {
 		// If the user can read the workspace proxy resource, return that.
 		// If not, always default to the regions.
@@ -677,6 +678,18 @@ func (api *API) updateEntitlements(ctx context.Context) error {
 		api.AGPL.AccessControlStore.Store(&acs)
 	}
 
+	if initial, changed, enabled := featureChanged(codersdk.FeatureAppearance); shouldUpdate(initial, changed, enabled) {
+		if enabled {
+			f := newAppearanceFetcher(
+				api.Database,
+				api.DeploymentValues.Support.Links.Value,
+			)
+			api.AGPL.AppearanceFetcher.Store(&f)
+		} else {
+			api.AGPL.AppearanceFetcher.Store(&appearance.DefaultFetcher)
+		}
+	}
+
 	// External token encryption is soft-enforced
 	featureExternalTokenEncryption := entitlements.Features[codersdk.FeatureExternalTokenEncryption]
 	featureExternalTokenEncryption.Enabled = len(api.ExternalTokenEncryption) > 0