Licensing: Send map of environment variables to plugins (#30347)

Replace hard-coded `GF_ENTERPRISE_LICENSE_TEXT` with a
`map[string]string` to allow the licensing service to allow sending any
environment variable through to the plugins for licensing purposes.

pkg/services/licensing/oss.go

@@ -60,7 +60,3 @@ func (l *OSSLicensingService) Init() error {
 func (*OSSLicensingService) HasValidLicense() bool {
 	return false
 }
-
-func (*OSSLicensingService) TokenRaw() string {
-	return ""
-}