support `current_timestamp` synonyms

[jycor]

In MySQL, these are all the same function:

• CURRENT_TIMESTAMP
• CURRENT_TIMESTAMP()
• NOW()
• LOCALTIME
• LOCALTIME()
• LOCALTIMESTAMP
• LOCALTIMESTAMP()

Dolt lacks support for these

dolt> select localtime;
function: 'localtime' not found
dolt> select localtime();
function: 'localtime' not found
dolt> select localtimestamp;
function: 'localtimestamp' not found
dolt> select localtimestamp();
function: 'localtimestamp' not found

Note: We also have slightly different implementations of NOW() and CURRENT_TIMESTAMP()

As a result, we lack support for using these functions as column defaults and as on update expressions.

dolt> create table t (ts timestamp default localtime);
function: 'localtime' not found

Note: We just support on update localtime, but treat it like a no-op

dolt> create table t (ts timestamp on update localtime);
dolt> show create table t;
+-------+------------------------------------------------------------------+
| Table | Create Table                                                     |
+-------+------------------------------------------------------------------+
| t     | CREATE TABLE `t` (                                               |
|       |   `ts` timestamp                                                 |
|       | ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin |
+-------+------------------------------------------------------------------+
1 row in set (0.00 sec)

Relevant MySQL Docs:

• https://dev.mysql.com/doc/refman/8.0/en/timestamp-initialization.html
• https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html

Related Issues:

• ON UPDATE CURRENT_TIMESTAMP in column definition does not update TIMESTAMP #6030
• current_timestamp() and now() do not return the same values #6058

[jycor]

Additionally, we should support parsing these synonyms in parenthesized on update expressions

create table t (ts timestamp on update (current_timestamp));

The above is a syntax error in MySQL; the correct behavior is not have SHOW CREATE TABLE print these expressions wrapped in parentheses.