supabase / supabase-flutter / 36532319957

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feat(postgrest): select several columns of an embed (#1890)

## Summary

Selecting two columns of the same relation on the typed surface, for
example `select([Books.author(Authors.id),
Books.author(Authors.name)])`, sent `author(id),author(name)`. PostgREST
joins the relation once per embed and Postgres rejects the repeated join
with 42803, so the request failed outright. One column per relation
worked.

This PR merges the entries of one relation into a single embed and adds
a way to select an embed as a whole.

## Changes

- **Merging.** Entries of one relation in a `select` list are rendered
as one embed at the position of the first entry, so the list above is
sent as `author(id,name)`. Merging recurses through nested embeds.
- **`PostgrestRelation.select`.** `Books.author.select([Authors.id,
Authors.name])` renders `author(id,name)` and `Books.author.select()`
renders `author(*)`. It returns a `PostgrestToOneEmbed` or
`PostgrestToManyEmbed`, both `PostgrestEmbed`, which is select position
only: not orderable, not filterable, no cast or aggregate. `select([])`
throws like the builder does. `columns` was not available as the method
name since the relation already carries its key columns under that name.
- **Sugar.** A single-column projection, `Books.author(Authors.name)`,
renders through the same path as a one-entry `select`, so there is one
code path to attach typed embedded rows to later
(https://linear.app/supabase/issue/SDK-1973/featpostgrest-type-the-nested-rows-of-an-embed-on-the-typed-surface).
- **`PostgrestSelectable`.** New sealed base of everything a `select`
list takes. `PostgrestColumnExpression` and `PostgrestEmbed` extend it,
and the typed `select` methods plus `PostgrestTableRequest.columns`
accept it.
- **Derivations inside embeds are structural.**
`PostgrestDerivedExpression` is now sealed over a plain derivation and
an embedded one that keeps its relation. A derivation of a nested
projection lands on the innermost column, `todo(or... (continued)

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