add param "pattern" for snowflake_external_table

[hkak03key]

Is your feature request related to a problem? Please describe.
I want to add the table which is created using spark and ignore the file _SUCCESS for that.

Describe the solution you'd like
add the param pattern for snowflake_external_table.

Describe alternatives you've considered
(none)

Additional context
It seems that pkg/snowflake/external_table.go is already implemented but pkg/resource/external_table.go was not.

I think the task is only to implement pkg/resource/external_table.go and test.

ref
#308
#309