Self-hosted in-product purchase (#21804)

* Self-hosted admins can purchase licenses in-app when `ServiceSettings,SelfHostedPurchase` is true (the default)
* Content Security Policy enables loading assets from `js.stripe.com/v3` when `ServiceSettings.SelfHostedPurchase` is true (the default).
* Add `hosted_customer` API subpath
* Add status of SelfHostedPurchase to telemetry config report.
* Support showing admins self-hosted invoices when `ServiceSettings.SelfHostedPurchase` is true (the default)

Co-authored-by: Conor Macpherson <116016004+ConorMacpherson@users.noreply.github.com>

einterfaces/cloud.go

@@ -33,7 +33,15 @@ type CloudInterface interface {
 	GetLicenseRenewalStatus(userID, token string) error
 	InvalidateCaches() error
 
+	// hosted customer methods
+	SelfHostedSignupAvailable() error
 	BootstrapSelfHostedSignup(req model.BootstrapSelfHostedSignupRequest) (*model.BootstrapSelfHostedSignupResponse, error)
+	CreateCustomerSelfHostedSignup(req model.SelfHostedCustomerForm, requesterEmail string) (*model.SelfHostedSignupCustomerResponse, error)
+	ConfirmSelfHostedSignup(req model.SelfHostedConfirmPaymentMethodRequest, requesterEmail string) (*model.SelfHostedSignupConfirmResponse, error)
+	ConfirmSelfHostedSignupLicenseApplication() error
+	GetSelfHostedInvoices() ([]*model.Invoice, error)
+	GetSelfHostedInvoicePDF(invoiceID string) ([]byte, string, error)
+
 	CreateOrUpdateSubscriptionHistoryEvent(userID string, userCount int) (*model.SubscriptionHistory, error)
 	HandleLicenseChange() error
 }