executor: add DROP ROLE support

[imtbkcat]

What problem does this PR solve?

add DROP ROLE function for parser and executor.

What is changed and how it works?

Role is same as user, is store in mysql.user table. We do same action as DROP USER when we need to drop a role. Also, as we has added some system table for RBAC, like mysql.role_edges and
mysql.default_role. We have to delete information from these tables when execute DROP ROLE

Check List

Tests

• Unit test

Code changes

• Has exported variable/fields change

Side effects

• Increased code complexity

[imtbkcat]

parser pr: pingcap/parser#237

[zhouqiang-cl]

/run-all-tests

[tiancaiamao]

Could drop role statement drop an user, or drop user statement drop an role in MySQL? @imtbkcat

[imtbkcat]

@tiancaiamao yes, drop role can drop user and drop user can also drop role.

[tiancaiamao]

LGTM

[jackysp]

pingcap/parser#237 is merged @imtbkcat

[imtbkcat]

/run-all-tests

[codecov]

Codecov Report

Merging #9616 into master will decrease coverage by 0.0368%.
The diff coverage is 21.875%.

@@               Coverage Diff                @@
##             master      #9616        +/-   ##
================================================
- Coverage   67.3912%   67.3544%   -0.0369%     
================================================
  Files           377        377                
  Lines         79353      79383        +30     
================================================
- Hits          53477      53468         -9     
- Misses        21104      21134        +30     
- Partials       4772       4781         +9

[imtbkcat]

PTAL @jackysp

[jackysp]

LGTM