Fix #369 - Add Support in Date/Time keywords

[yuval-illumex]

Fix #369

Add Support for:
CURRENT_TIMESTAMP
CURRENT_DATE
CURRENT_TIME

Which are function without args.

[alamb]

I double checked that both mysql and postgres indeed do treat these specially:
👍
Thanks @yuval-illumex

mysql> select current_timestamp;
+---------------------+
| current_timestamp   |
+---------------------+
| 2021-12-17 12:30:40 |
+---------------------+
1 row in set (0.00 sec)

mysql> select current_time;
+--------------+
| current_time |
+--------------+
| 12:30:45     |
+--------------+
1 row in set (0.00 sec)

mysql> select current_date;
+--------------+
| current_date |
+--------------+
| 2021-12-17   |
+--------------+
1 row in set (0.00 sec)

mysql> ^DBye
(arrow_dev) alamb@MacBook-Pro-2:~/Software/arrow-datafusion$ psql
psql (14.1)
Type "help" for help.

alamb=# select current_timestamp;
       current_timestamp       
-------------------------------
 2021-12-17 12:30:59.547784-05
(1 row)

alamb=# select current_time;
    current_time    
--------------------
 12:31:02.464701-05
(1 row)

alamb=# select current_date;
 current_date 
--------------
 2021-12-17
(1 row)

[coveralls]

Pull Request Test Coverage Report for Build 1616040264

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 37 of 39 (94.87%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 90.252%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	9	11	81.82%

Totals
Change from base Build 1615803810:	0.03%
Covered Lines:	6703
Relevant Lines:	7427

---

💛 - Coveralls

[alamb]

Looks good -- thanks @yuval-illumex

[alamb]

These appear to have been included in #391 so closing this one down