Add pg_encoding_set_invalid()

There are cases where we cannot / do not want to error out for invalidly
encoded input. In such cases it can be useful to replace e.g. an incomplete
multi-byte characters with bytes that will trigger an error when getting
validated as part of a larger string.

Unfortunately, until now, for some encoding no such sequence existed. For
those encodings this commit removes one previously accepted input combination
- we consider that to be ok, as the chosen bytes are outside of the valid
ranges for the encodings, we just previously failed to detect that.

As we cannot add a new field to pg_wchar_table without breaking ABI, this is
implemented "in-line" in the newly added function.

Author: Noah Misch <noah@leadboat.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Backpatch-through: 13
Security: CVE-2025-1094

src/test/regress/sql/conversion.sql

@@ -8,6 +8,11 @@
 
 \set regresslib :libdir '/regress' :dlsuffix
 
+CREATE FUNCTION test_enc_setup() RETURNS void
+    AS :'regresslib', 'test_enc_setup'
+    LANGUAGE C STRICT;
+SELECT FROM test_enc_setup();
+
 CREATE FUNCTION test_enc_conversion(bytea, name, name, bool, validlen OUT int, result OUT bytea)
     AS :'regresslib', 'test_enc_conversion'
     LANGUAGE C STRICT;