Support create index with clause

[lewiszlw]

See postgresql doc: https://www.postgresql.org/docs/current/sql-createindex.html

[iffyio]

LGTM! cc @alamb

[coveralls]

Pull Request Test Coverage Report for Build 10653659255

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 62 of 68 (91.18%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 89.251%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/dialect/generic.rs	1	2	50.0%
src/dialect/mod.rs	1	2	50.0%
src/dialect/postgresql.rs	1	2	50.0%
src/parser/mod.rs	8	9	88.89%
tests/sqlparser_common.rs	35	37	94.59%

Totals
Change from base Build 10653580269:	0.004%
Covered Lines:	28737
Relevant Lines:	32198

---

💛 - Coveralls

[alamb]

Thank you @lewiszlw @iffyio and @git-hulk

I pushed a commit to this PR to fix the CI doc failure too

[alamb]

Thanks again !