feat: support variable create / update methods and text attribute

[ludoo]

This PR adds support for variable create and update methods.

The create method has been implemented in the Variable class instead of Config (which would have given symmetry with get_variable) for a couple of reasons:

• it allows a more natural workflow of calling config.variable(), setting the desired attribute, then saving
• it updates the created variable in place via _set_properties like other variable methods

This also adds support for the text attribute, which can now be used in alternative to the previously supported value. The create and update methods enforce mutual exclusivity of the two attributes.

Fixes #1

[software-dov]

Looks good, just one minor nit.

[ludoo]

@software-dov sorry for the extra round, I did not consider the linting step in the presubmit, can you re-run kokoro?

As an aside, it might be worth documenting the steps one can use to replicate the presubmit in the CONTRIBUTING doc, to simplify contributions and waste less time for reviewers. I can do that after if you think it useful.