PL/pgSQL: Add support for the ALIAS statement

[fulghum]

Adds support for the execution logic for the ALIAS statement in user defined functions.

[github-actions]

	Main	PR
covering_index_scan_postgres	354.83/s	361.92/s	+1.9%
index_join_postgres	152.61/s	153.63/s	+0.6%
index_join_scan_postgres	184.47/s	183.24/s	-0.7%
index_scan_postgres	12.64/s	12.62/s	-0.2%
oltp_point_select	2773.40/s	2796.86/s	+0.8%
oltp_read_only	1882.04/s	1869.73/s	-0.7%
select_random_points	111.16/s	110.69/s	-0.5%
select_random_ranges	124.11/s	127.10/s	+2.4%
table_scan_postgres	11.91/s	11.92/s	0.0%
types_table_scan_postgres	5.65/s	5.58/s	-1.3%

[github-actions]

	Main	PR
Total	42090	42090
Successful	15537	15537
Failures	26553	26553
Partial Successes1	5226	5226

	Main	PR
Successful	36.9138%	36.9138%
Failures	63.0862%	63.0862%

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. ↩

[Hydrocharged]

LGTM! I apologize for having to manually write out the example function's operations, it's relatively straightfoward but a pain.