Add support for ALTER TABLE ... DROP CONSTRAINT IF EXISTS

[fulghum]

Depends on: dolthub/go-mysql-server#2936
Depends on: dolthub/vitess#411

[github-actions]

	Main	PR
covering_index_scan_postgres	336.31/s	338.17/s	+0.5%
index_join_postgres	152.71/s	154.83/s	+1.3%
index_join_scan_postgres	183.89/s	185.12/s	+0.6%
index_scan_postgres	12.63/s	12.67/s	+0.3%
oltp_point_select	2660.90/s	2694.66/s	+1.2%
oltp_read_only	1850.79/s	1841.99/s	-0.5%
select_random_points	115.59/s	116.05/s	+0.3%
select_random_ranges	128.75/s	127.24/s	-1.2%
table_scan_postgres	12.22/s	12.10/s	-1.0%
types_table_scan_postgres	5.64/s	5.63/s	-0.2%

[github-actions]

	Main	PR
Total	42090	42090
Successful	16316	16317
Failures	25774	25773
Partial Successes1	5522	5522

	Main	PR
Successful	38.7646%	38.7669%
Failures	61.2354%	61.2331%

${\color{lightgreen}Progressions (1)}$

alter_table

QUERY: alter table anothertab drop constraint IF EXISTS anothertab_chk;

Footnotes

1. These are tests that we're marking as Successful, however they do not match the expected output in some way. This is due to small differences, such as different wording on the error messages, or the column names being incorrect while the data itself is correct. ↩

[zachmu]

LGTM