Add initial support for secrets

The functions "<Connection>.ListSecrets", "<Secret>.Free" are now
available.

Signed-off-by: Crístian Deives <cristiandeives@gmail.com>
Add secret functions "Define" and "Undefine"

The functions "<Connection>.DefineSecret", "<Secret>.Undefine"
are now available.

Signed-off-by: Crístian Deives <cristiandeives@gmail.com>
Add secret getter functions

The functions "<Secret>.UUID", "<Secret>.XML", "<Secret>.UsageID" and
"<Secret>.UsageType" are now available.

Signed-off-by: Crístian Deives <cristiandeives@gmail.com>
Add secret value functions

The functions "SetValue", "Value" are now available.

Signed-off-by: Crístian Deives <cristiandeives@gmail.com>
Add secret lookup functions

The functions "<Connection>.LookupSecretByUUID" and
"<Connection>.LookupSecretByUsage" are now available.

Signed-off-by: Crístian Deives <cristiandeives@gmail.com>
Add secret function "Ref"

The function "<Secret>.Free" is now available.

Signed-off-by: Crístian Deives <cristiandeives@gmail.com>