Add field:create command

[DieterHolvoet]

This PR adds a command to create fields, similar to the field-create command that was present in Drush 8. It has already been used in multiple projects at the company I work at, and is in my opinion stable enough to be considered to be merged in Drush.

Tests

I do believe that this command should be backed by tests, but I have no past experience with PhpUnit and I can't seem to get the existing tests passing on my local machine, even without my changes.

Naming

I named the class FieldCreateCommands instead of FieldCommands because I think future (smaller) field-related commands should be kept seperate from the field-create command. The class is already almost 650 lines long and adding other commands would make it a bit too cluttered.

[weitzman]

Yes, lets add tests before merging. We have a problem with tests that just arose, as you can see on the results of this PR.

[weitzman]

Lets skip this command, as we now have generate field which is somewhat similar.

[DieterHolvoet]

I just tried out generate field and it does something completely different. This command adds a new field to an entity type, it doesn't create a new field type.