Parser: Add support for the DEFAULT ROLE clause in CREATE USER and ALTER USER statements

[joshvarner]

Feature Request

Is your feature request related to a problem? Please describe:

Currently the parser fails to handle valid MySQL statements, such as:

CREATE USER foo DEFAULT ROLE bar;

This has downstream effects with libraries & tools like go-mysql/canal that will fail to handle these statements when a default role is specified.

Describe the feature you'd like:

The parser should be updated to support the DEFAULT ROLE clause in both CREATE USER and ALTER USER statements.