feat: adds .explain() for debugging performance issues on Supabase client generated queries.

[dshukertjr]

What kind of change does this PR introduce?

Adds a new .explain() transformer that will return

final String plan = await supabase.from('table').select().explain(); 

// `plan` will be something like this as plain String:
// Aggregate  (cost=8.18..8.20 rows=1 width=112)
//  ->  Index Scan using projects_pkey on projects  (cost=0.15..8.17 rows=1 width=40)
//        Index Cond: (id = 1)

The JS SDK has a json and text response types, but text is preferred, so I think it's okay to only have text for Dart SDK.

JS implementation here https://github.com/supabase/postgrest-js/blob/master/src/PostgrestTransformBuilder.ts#L245

Closes #717