Add support for CASE ... WHEN in PL/pgSQL

[fulghum]

No description provided.

[github-actions]

	Main	PR
covering_index_scan_postgres	360.87/s	349.85/s	-3.1%
index_join_postgres	152.72/s	152.83/s	0.0%
index_join_scan_postgres	183.27/s	183.56/s	+0.1%
index_scan_postgres	11.91/s	12.09/s	+1.5%
oltp_point_select	2774.17/s	2827.13/s	+1.9%
oltp_read_only	1848.10/s	1874.98/s	+1.4%
select_random_points	110.11/s	110.46/s	+0.3%
select_random_ranges	129.65/s	128.51/s	-0.9%
table_scan_postgres	9.84/s	10.10/s	+2.6%
types_table_scan_postgres	5.12/s	5.21/s	+1.7%

[github-actions]

	Main	PR
Total	42090	42090
Successful	15684	15684
Failures	26406	26406
Partial Successes1	5209	5209

	Main	PR
Successful	37.2630%	37.2630%
Failures	62.7370%	62.7370%

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!