fix(snowflake): COPY postfix

[VaggelisD]

Fixes #3388

Design notes

• Snowflake's FILE_PARAM option is now parsed & generated on its own to accommodate for the optional formatTypeOptions that might come after the TYPE opts:

FILE_FORMAT = (  
 FORMAT_NAME = '[<namespace>.]<file_format_name>' | 
 TYPE = { CSV | ... | XML } [ formatTypeOptions ] 
) 

• Allowed parse_primary() to parse expressions such as NULL_IF = () into empty tuples. That would otherwise try to be parsed in an exp.Paren with no expression, which would throw an error

• Refactored the file parsing to also include Snowflake's @external_file syntax besides strings

• Added the issue query and other Snowflake doc examples as test cases

Docs

• Snowflake COPY

[georgesittas]

Will review as well shortly

[georgesittas]

Thanks for addressing comments @VaggelisD 👍