[FEATURE] Better support for class member as extensions

[goodwanghan]

Is your feature request related to a problem? Please describe.
The piece of code describes the problem

import pandas as pd
from fugue_sql import FugueSQLWorkflow

class Test(object):
    # schema: *
    def t(self, df:pd.DataFrame) -> pd.DataFrame:
        return df
    
test = Test()

with FugueSQLWorkflow() as dag:
    dag.df([[0]],"a:int").transform(test.t).show() # this works

f = test.t

with FugueSQLWorkflow() as dag:
    dag("""
    CREATE [[0]] SCHEMA a:int
    TRANSFORM USING f   # this works
    PRINT
    """)
    
with FugueSQLWorkflow() as dag:
    dag("""
    CREATE [[0]] SCHEMA a:int
    TRANSFORM USING test.t   # this complains test.t is not a valid transformer
    PRINT
    """)

Describe the solution you'd like
TRANSFORM USING test.t should work, test.t should be recognized
This is not a problem of fugue, but a problem of triad
https://github.com/fugue-project/triad/blob/84d1f01be413251a5a85ce89025c982e1306ed70/triad/utils/convert.py#L155
It needs improvement in triad to be able to recognize this expression.
On Fugue side, additional tests should be added to test class member as extension scenarios

Describe alternatives you've considered
The alternative is the second example this the code above

Additional context
NA

[goodwanghan]

fugue-project/triad#52

[goodwanghan]

The Triad change is causing behavioral change on string parsing, so this becomes a high priority item to fix