Codegen panics when SQL aliases are not valid Rust identifiers

[Micah Wylde]

For example, this query:

SELECT 'a' as "1"
from nexmark;

produces a panic:

ERROR arroyo_server_common: panicked at 'Ident cannot be a number; use Literal instead', /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.66/src/fallback.rs:756:9 panic.file="/root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.66/src/fallback.rs" panic.line=756 panic.column=9