pgxn does not (yet) support the notion of TLEs, but it certainly could with minimal changes. We're currently in discussion with the pgxn maintainers about how that would look.
RDS for Postgres 14+ does come with pg_tle, but it does not come with pgsql-http, so you cannot use the dbdev installer. But, you can still install TLEs, you just have to do so "by hand" by calling `pgtle.install_extension()` yourself, probably from a migration script depending on your tooling.