sql: modify TRIM() function signature

According to the ANSI standard, ltrim, rtrim and trim should
be merged into one unified TRIM() function. The specialization of
trimming (left, right or both and trimming characters) determined
in arguments of this function.

Closes #3879

@TarantoolBot document
Title: TRIM() function

Modify signature of SQL function TRIM(). This function removes
characters included in <trim character> (binary) string from
<trim source> (binary) string until encounter a character that doesn't
belong to <trim character>. Removal occurs on the side, specified by
<trim specification>. Now, syntax is following:
TRIM([ [ <trim specification> ] [ <trim character> ] FROM ] <trim source>).

<trim specification> can be one of the following keywords: LEADING,
TRAILING and BOTH.
<trim character> is the set of trimming characters.
<trim source> is the string, that will be trimmed.
If FROM is specified, then:
1) Either <trim specification> or <trim character> or both shall be
specified.
2) If <trim specification> is not specified, then BOTH is implicit.
3) If <trim character> is not specified, then ' ' is implicit.