Allow a type alias for `TableIterator` in `#[pg_extern]`.

[if0ne]

Code:

pub type Output = TableIterator<'static, (name!(bar, String), name!(baz, i32))>;

#[pg_extern]
pub fn test(foo: String) -> Output {
    Output::empty()
}

Run cargo run pgrx

Error:

thread 'main' panicked at /tmp/.tmpp9lY5M:1:12155:
Could not write SQL to /home/pagafonov/.pgrx/13.20/pgrx-install/share/postgresql/extension/pgnats--0.1.0.sql: Got non-plain mapped/composite return variant SQL in what macro-expansion thought was a type, got: Table([As("TEXT"), As("INT")])

Stacktrace:

Location:
    /home/pagafonov/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pgrx-sql-entity-graph-0.14.1/src/pg_extern/entity/mod.rs:185:45
stack backtrace:
   0: rust_begin_unwind
             at /rustc/05f9846f893b09a1be1fc8560e33fc3c815cfecb/library/std/src/panicking.rs:695:5
   1: core::panicking::panic_fmt
             at /rustc/05f9846f893b09a1be1fc8560e33fc3c815cfecb/library/core/src/panicking.rs:75:14
   2: core::result::unwrap_failed
             at /rustc/05f9846f893b09a1be1fc8560e33fc3c815cfecb/library/core/src/result.rs:1704:5
   3: core::result::Result<T,E>::expect
             at /home/pagafonov/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/result.rs:1061:23
   4: pgrx_embed_pgnats::pgrx_embed::main
             at /tmp/.tmpp9lY5M:1:12044
   5: core::ops::function::FnOnce::call_once
             at /home/pagafonov/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.