Add a Row-like interface to any PostgreSQL composite type

[alexwl]

It can be useful to be able to get fields of a composite type without the need to derive FromSql for a structure (#366).

This PR adds a new type CompositeType that implements FromSql and has the same methods as Row.

Example:

create table users (
  id               serial primary key,
  name             text
);
create table posts (
  id               serial primary key,
  author_id        integer not null references users(id),
  title            text,
  content          text
);

let query = "SELECT users.*,ARRAY_REMOVE(ARRAY_AGG(posts),NULL) as posts FROM users LEFT JOIN posts ON posts.author_id = users.id GROUP BY users.id";
let stmt = client.prepare(query).await?;
let rows: Vec<Row> = client.query(&stmt, &[]).await?;
for user_with_posts in rows {
    let user_name = user_with_posts.get::<_,&str>("name");
    // ...
    let user_posts = user_with_posts.get::<_,Vec<CompositeType>>("posts");
    for post in user_posts {
        let post_title = post.get::<_,&str>("title");
        let post_content = post.get::<_,&str>("content");
        // ...
    }
}

The binary format of a composite type is different from the format of a DataRow (there is an OID before each length/value pair), so I added a type parameter to DataRowRanges to make FallibleIterator for ranges usable for both cases:
https://github.com/alexwl/rust-postgres/blob/aa4b5547e3bf13e112ae90dd919b0ac70d67ebaa/postgres-protocol/src/message/backend.rs#L550-L574

[sfackler]

This module is specifically for parsing the backend protocol messages. I'd rather have composite support added in the types module.

[sfackler]

It seems like this can just be inlined in to the FromSql implementation.

[alexwl]

I moved the code from CompositeType::new to FromSql::from_sql and moved the iterator over the fields of a composite type to postgres-protocol/src/types/mod.rs.

[runfalk]

Is there something blocking this? It is something I'm quite interested in myself.

[alexwl]

I'm also still interested in this. I can update the PR if necessary.

[mpalmer]

I've rebased this PR against current master, and it applied successfully (patch seemed to find the right offsets). I'm now using it in a project, and so far it seems to working great, so I'd be 👍 for merge. One thing that seemed a bit odd was how CompositeType was in tokio-postgres, rather than, say, postgres-types, as I can't see any async-specific code in the implementation. (ETA: merged code is at https://github.com/mpalmer/rust-postgres branch pr565 if anyone wants to use it)

[phillipleblanc]

If you need this before its merged, you can also just copy the structs from this PR into your codebase to use it without needing to fork this library. An example of this can be found here: https://github.com/spiceai/spiceai/blob/trunk/crates/arrow_sql_gen/src/postgres/composite.rs