refactor!(postgrest, supabase): make schema variable private and rename useSchema() to schema()

[dshukertjr]

What kind of change does this PR introduce?

Currently the JS library uses .schema() to change the schema for calling APIs to non-public schema. However, the Dart library uses useSchema(), because schema symbol was already taken by the schema variable in postgrest.dart.

This PR renames the schema variable to _schema and .useSchema() to .schema() to align the public API with the JS library.

What is the current behavior?

await supabase.useSchema('custom_schema').from('table').select();

What is the new behavior?

await supabase.schema('custom_schema').from('table').select();