[SPARK-29941][SQL] Add ansi type aliases for char and decimal

[yaooqinn]

What changes were proposed in this pull request?

Checked with SQL Standard and PostgreSQL

CHAR is equivalent to CHARACTER. DEC is equivalent to DECIMAL. INT is equivalent to INTEGER. VARCHAR is equivalent to CHARACTER VARYING. ...

postgres=# select dec '1.0';
numeric
---------
1.0
(1 row)

postgres=# select CHARACTER '. second';
  bpchar
----------
 . second
(1 row)

postgres=# select CHAR '. second';
  bpchar
----------
 . second
(1 row)

Why are the changes needed?

For better ansi support

Does this PR introduce any user-facing change?

yes, we add character as char and dec as decimal

How was this patch tested?

add ut

[yaooqinn]

cc @cloud-fan @maropu, pre-discussion @ #26537 (comment), thanks.

[SparkQA]

Test build #114012 has finished for PR 26574 at commit ea5b5a4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[cloud-fan]

thanks, merging to master!