eca_content: add action getfieldvalue
Problem/Motivation
Unless I'm missing something, we do have the action eca_set_field_value but not the equivalent eca_get_field_value. At a glance, it looks like the token system is given us that already, but there are 2 scenarios where that doesn't help: if the contrib token module is not installed and for fields that are not even exposed by tokens.
The latter was a problem when building an example model for a user on slack who needed access to the data array of a commerce order. The same could happen if one wanted to access user data, which doesn't seem to be accessible through tokens either.
Proposed resolution
Should be implemented just like SetFieldValue, but of course in the opposite direction. Ideally, this should go into 1.0 as well but I'm not tagging it yet until we conclude that this is the right way to go.