fix: avoid "Unable to determine type" warning with JSON columns in to_dataframe

[tswast]

TODO:

• tests
• maybe we don't want string columns for JSON?

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Fixes #<issue_number_goes_here> 🦕

[tswast]

I might actually want to do something in db-dtypes so that even though it's a string the unboxed version would give a parsed object like the behavior is when the REST API is used.

[tswast]

Right now the behavior is inconsistent across REST and BQ Storage API.

[tswast]

Marking as do not merge for now, as this makes JSON dtype consistent now but always return string dtype like the BQ Storage Read API code path does, which isn't ideal.