feat: Adding EXCLUDE constraint support

[VaggelisD]

Fixes #3097

The syntax for the EXCLUDE table constraint is the following:

[CONSTRAINT constraint_name] EXCLUDE [ USING index_method ] ( exclude_element WITH operator [, ... ] ) index_parameters [ WHERE ( predicate ) ]

where:

<index_method> ::= identifier (see [1])
<exclude_element> ::= { column_name | ( expression ) } [ opclass ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ]
<opclass> ::= identifier (see [2])
<index_parameters> ::=  [ INCLUDE ( column_name [, ... ] ) ] 
                        [ WITH ( storage_parameter [= value] [, ... ] ) ]
                        [ USING INDEX TABLESPACE tablespace_name ]

Design Notes

• Added exp.ExcludeElement that captures all information from exclude_element WITH operator [, ... ]
• This is an exclusively Postgres feature afaik, but the changes are added on the base Dialect.

Docs

1. Index methods
2. Operator class
3. EXCLUDE constraint

[georgesittas]

Looks good, nice job! Can we get rid of that lstrip() call somehow?