Move virSecretObj from secret_driver.c to virsecretobj.h

To support being able to create a hashed secrets list, move the
virSecretObj to virsecretobj.h so that the code can at least find
the definition.

This should be a temporary situation while the virsecretobj.c code
is patched in order to support a hashed secret object while still
having the linked list support in secret_driver.c. Eventually, the
goal is to move the virSecretObj into virsecretobj.c, although it
is notable that the existing model from which virSecretObj was
derived has virDomainObj in src/conf/domain_conf.h and virNetworkObj
in src/conf/network_conf.h, so virSecretObj wouldn't be unique if
it were to remain in virsecretobj.h  Still adding accessors to fetch
and store hashed object data will be the end goal.

Add definitions and infrastucture in virsecretobj.c to create and
handle a hashed virSecretObj and virSecretObjList including the class,
object, lock setup, and disposal API's. Nothing will call these yet.

This infrastructure will replace the forward linked list logic
within the secret_driver, eventually.