Implement a feature-based geometry lock mechanism

[nirvn]

This PR implements a functionality in qfieldsync allowing users to determine the locked geometry state of individual features within a vector layer.

First, here's a screenshot of the UI:

A brand new data-defined property button sits next to the [x] lock geometry checkbox. From there, users can select a feature attribute or build a more complex expression that will change the locked geometry status of individual features.

(Requires opengisch/libqfieldsync#58 on the libqfieldsync side of things)

[suricactus]

Two comments on the code, just clarifications on naming. Very nice it is using the same mechanism to define dynamic value as in the rest of QGIS.