[FEATURE] Allow to set tx_extbase_type via TS with forceValues

To be able to use forceValues for the field tx_extbase_type
a matching function setTxExtbaseType() must exist.
Since Extbase does not provide it, add it to the extended
user-model of femanager instead.

Change-Id: I3d757cf37edf5802166626c1379fa6158f086adb
Resolves: #58392
Reviewed-on: https://review.typo3.org/29760
Reviewed-by: Alex Kellner
Tested-by: Alex Kellner