sql: fix CREATE TABLE AS sourcing SHOW <show_subcmd> <table> job failures

[ecwall]

Fixes #106260

Previously CREATE TABLE AS/CREATE MATERIALIZED VIEW AS sourcing from SHOW <show_subcmd> <table> generated a failing schema change job with a relation "tbl" does not exist error because the SHOW source table was not fully qualified.

This PR fixes this by respecting the Builder.qualifyDataSourceNamesInAST flag in delegate.TryDelegate() which implements the failing SHOW commands.

Release note (bug fix): Fix failing schema change job when CREATE TABLE AS or CREATE MATERAILIZED VIEW AS sources from a SHOW command:

1. CREATE TABLE t AS SELECT * FROM [SHOW CREATE TABLE tbl];
2. CREATE TABLE t AS SELECT * FROM [SHOW INDEXES FROM tbl];
3. CREATE TABLE t AS SELECT * FROM [SHOW COLUMNS FROM tbl];
4. CREATE TABLE t AS SELECT * FROM [SHOW CONSTRAINTS FROM tbl];
5. CREATE TABLE t AS SELECT * FROM [SHOW PARTITIONS FROM TABLE tbl];
6. CREATE TABLE t AS SELECT * FROM [SHOW PARTITIONS FROM INDEX tbl@tbl_pkey];

[cockroach-teamcity]

This change is 

[chengxiong-ruan]

LGTM.

[ecwall]

bors r=chengxiong-ruan

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

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