supabase / supabase-flutter / 33505617846

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feat: add supabase_typegen package generating typed table definitions (#1635)

## What kind of change does this PR introduce?

Feature (draft, layer 2 of the typed table access work, stacked on
#1634). Adds a new `supabase_typegen` package: a standalone code
generator that turns a database schema into the typed table definitions
introduced in #1634, so users get the fully typed surface without
writing any of it by hand.

Linear: SDK-1362

## What is the new behavior?

```sh
supabase gen types --lang dart --local > lib/supabase_schema.g.dart
```

The CLI runs postgrest-typegen's introspection in-process against the
database and hands the language-neutral `GeneratorMetadata` document,
the intermediate representation of
[`@supabase/postgrest-typegen`](https://github.com/supabase/sdk/tree/main/packages/postgrest-typegen)
that its TypeScript, Go, Swift, and Python generators also consume, to
this tool over stdin. The tool emits one Dart file containing, per
table:

- a zero-cost row extension type over the decoded JSON map with typed
getters (`DateTime` parsing, `double`/`num` coercion, `List` casts,
Postgres enum mapping),
- `Insert` and `Update` value extension types that implement
`Map<String, dynamic>`, with required parameters derived from `NOT
NULL`-without-default columns and null-aware omission for everything
else; explicit SQL NULL writes go through generated `set…ToNull` copy
methods that only exist for nullable, writable columns,
- a `PostgrestTable` definition plus `TableColumn` tokens for
compile-time checked filters,
- Dart enums for Postgres enums with wire-name mapping (`toString`
returns the wire name so enum values work directly in filters).

See `packages/supabase_typegen/test/goldens/supabase_schema.dart` for
what the output looks like for the fixture schema.

Design choices worth reviewing:

- **Introspection source**: the `GeneratorMetadata` contract of
`@supabase/postgrest-typegen` (version 1, as shipped in the released
0.2.0 and emb... (continued)

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