Support strings

[pearu]

Perhaps already have string dictionary support by inputting key_for_string, but could make nicer ux around this

[pearu]

See Slack discussions for ideas:

1. https://quansight.slack.com/archives/C01CAU0U1FH/p1605526178096400

[pearu]

Related PRs:

• https://github.com/omnisci/omniscidb-internal/pull/5492
• Add test for dict encoded string types - omniscidb-internal PR 5492 #322

[guilhermeleobas]

Related PR:

• Add test for dict encoded string types - omniscidb-internal PR 5492 #322

[guilhermeleobas]

TextEncodingNone and TextEncodingDict types are supported.

[pearu]

Tests in rbc/tests/heavyai/test_text_encoding.py cover the cases where TextEncodingDict variables are passed around but we don't have tests for accessing the content of TextEncodingDict variables. In short, we should be able to define a UDF that translates a TextEncodingDict variable to TextEncodingNone return value, for instance. And vice-versa, checking if TextEncodingNone variable can be converted to TextEncodingDict return value (I imagine if the input string is not in the strings dictionary, the conversion should fail, or add a new item to the dict).

[guilhermeleobas]

PR #531 adds support for python strings