feat: Supporting SAMPLE parsing

[seve-martinez]

Snowflake, Databricks, Postgres, and others all support some from of table sampling.

This commonly takes on the form

SELECT ...
FROM ... 
{ SAMPLE | TABLESAMPLE } [ samplingMethod ]

where

samplingMethod ::= { { BERNOULLI | ROW } ( { <probability> | <num> ROWS } ) |
                     { SYSTEM | BLOCK } ( <probability> ) [ { REPEATABLE | SEED } ( <seed> ) ] }

Different dialects support one, the other, or both sample keywords and only some of the methods. The current PR follows the snowflake paradigm.

Outstanding questions

• This method is not currently called anywhere. Review of the Parser code would make me think it should be part of the parse_table_factor() logic since it always trails a relation. However this would then require each variant of TableFactor that supports SAMPLE to have a placeholder for it. Is that the right approach?

[iffyio]

Hi @seve-martinez! It sounds like the parsing can be done while parsing the select body instead? Probably here before looking for a WHERE clause?