Add optional serialization_type and hmac_key param values to RESTBagOStuff

Add optional serialization_type and hmac_key param values to RESTBagOStuff

T233537 made RESTBagOStuff work with the Kask external session
storage service, but broke backward compatibility. Add optional
values to the RESTBagOStuff $params constructor parameter to
support communicating with Kask, and to allow using HMAC. The
new values are:
  serialization_type: legacy (default), PHP, or JSON
  hmac_key: HMAC key to use for protecting the serialized blob
If these new values are not specified, behavior remains
unchanged (PHP serialization with no HMAC protection).

Bug: T233963
Change-Id: Ia2625c04e08cfe9616569500f1d613be73c170a2
(cherry picked from commit 2ed69af15cb8971e35aafc2c08397ed38c480488)