Add support for extended TRIM function grammar

[theodesp]

Feature Description

We would like to utilize the TRIM grammar as defined in MySQL
https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_trim

Currently, I can only use the basic version of TRIM and I couldn't find anything in the yacc file
https://github.com/vitessio/vitess/blob/a78587406b25f795429740f5426695681fc252ad/go/vt/sqlparser/sql.y

Use Case(s)

In relevance to implementing the Trim function in https://github.com/src-d/go-mysql-server we are using Vitess to parse the SQL rules.