Replace the two booleans from 'SET ROLE' with a single enum ContextModifier

[AugustoFKL]

This way, we don't rely on the parser to have valid structures for that statement, as the structure itself is valid.

Postgresql doesn't allow both to be present (1), so there's no meaning in having a structure that allows both. Also, ANSI (2) and a lot of other dialects support that command, so having the comment PostgreSQL specific doesn't make sense.

@alamb This is ready for review. Also, do you know why the coverage is so weird? Like, a lot of enum matches is not covered, even though their interior is

[1] : https://www.postgresql.org/docs/current/sql-set-role.html
[2] : https://jakewheat.github.io/sql-overview/sql-2016-foundation-grammar.html#set-role-statement

[coveralls]

Pull Request Test Coverage Report for Build 3215185150

• 24 of 28 (85.71%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 85.794%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/mod.rs	8	12	66.67%

Totals
Change from base Build 3209998484:	0.005%
Covered Lines:	10484
Relevant Lines:	12220

---

💛 - Coveralls

[alamb]

Makes sense to me -- thank you @AugustoFKL