[no-release-notes] Implement IsStringType for DoltgresType

[angelamayxie]

depends on dolthub/go-mysql-server#3343

[github-actions]

	Main	PR
covering_index_scan_postgres	542.54/s	535.65/s	-1.3%
index_join_postgres	125.69/s	122.56/s	-2.5%
index_join_scan_postgres	183.98/s	184.27/s	+0.1%
index_scan_postgres	11.48/s	11.50/s	+0.1%
oltp_point_select	2385.86/s	2400.68/s	+0.6%
oltp_read_only	1725.04/s	1734.07/s	+0.5%
select_random_points	110.56/s	110.28/s	-0.3%
select_random_ranges	42.13/s	43.11/s	+2.3%
table_scan_postgres	11.25/s	11.36/s	+0.9%
types_table_scan_postgres	5.24/s	5.19/s	-1.0%

[github-actions]

	Main	PR
Total	42090	42090
Successful	17304	17304
Failures	24786	24786
Partial Successes1	5596	5596

	Main	PR
Successful	41.1119%	41.1119%
Failures	58.8881%	58.8881%

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