Convert Bigquery tags to standard format, compatible (at least) with AWS glue tags.

[rsyi]

Given our recent discussion in slack, ideally we should adjust the format in which Bigquery saves tags to the TableMetadata model to have a consistent format between both Glue and Bigquery.

Let's do this after merging https://github.com/dataframehq/whale/pull/93.

@manuelzander Would love your input here -- it seems like there are mostly just key value pairs in Bigquery tags, but there's also the "template" and "templateDisplayName". Are these necessary to surface? Perhaps it could just suffice to show the fields dict?