Add basic auth to schema registry

[claudio-benfatto]

This PR adds the capability to use HTTP basic authentication to connect to the schema registry.

An example of a client using this code would be:

var schemaRegistryClient schema.SchemaRegistryClientInterface
schemaRegistryClient = schema.NewCachedSchemaRegistryClient([]string{*registry})

if *registryBasicAuth != "" {
  username := *registryUser
  password := *registryPassword
  schema.BasicAuth(registryUser, registryPassword)(schemaRegistryClient)
}