plan, privilege: add role support for SHOW GRANT

[imtbkcat]

What problem does this PR solve?

1. Support display roles for certain user in SHOW GRANT.
2. Support display privileges brought by roles for certain user in SHOW GRANT USING roles

What is changed and how it works?

1. Add Roles field for ShowStmt, ShowExec.
2. When executing SHOW GRANT, reading roles for current user from roleGraph.
3. When searching privilege tables like TablesPriv and DbPriv, not only matching record with current user, but also with roles in Roles field.

Some comment change was caused by goword check before commit.
It will block the commit if I don't chenge these comment.

Check List

Tests

• Unit test

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change

Side effects

• Increased code complexity

[imtbkcat]

parser pr: pingcap/parser#272

[imtbkcat]

/run-all-tests

[codecov]

Codecov Report

Merging #10016 into master will increase coverage by 0.0152%.
The diff coverage is 92.2222%.

@@               Coverage Diff                @@
##             master     #10016        +/-   ##
================================================
+ Coverage   77.9584%   77.9736%   +0.0152%     
================================================
  Files           407        407                
  Lines         83075      83146        +71     
================================================
+ Hits          64764      64832        +68     
  Misses        13516      13516                
- Partials       4795       4798         +3

[jackysp]

LGTM
PTAL @tiancaiamao

[tiancaiamao]

LGTM
Please resolve conflicts @imtbkcat

[imtbkcat]

/run-all-tests

[imtbkcat]

PTAL @tiancaiamao

[tiancaiamao]

/run-code_coverage

[zhouqiang-cl]

/rebuild