Acquire list of keywords from CrateDB

[amotl]

There is no remote command to inquire the list of keywords from CrateDB itself yet, right?

select * from pg_get_keywords()

Not all data types are automatically keywords. In fact, most aren't.

Looks like the list of keywords in the admin-ui already contains a lot of things that are not keywords in CrateDB.

Originally posted by @mfussenegger in #724 (comment)

[proddata]

with #731 datatypes would be handled separately from reserved keywords.