Allow use of LLRT runtime

[nerdoza]

AWS released an experimental runtime called LLRT (Low Latency Runtime) which promises to be faster at cold boots than even some of the compiled runtimes containers like Go. The full details are here: https://github.com/awslabs/llrt

As an experiment, sst should support using the LLRT runtimes. The best place to start would likely be with the Function construct, and then if it's stable and workable apply the changes to SsrFunction construct as well.

The runtime should be an opt-in only runtime for the foreseeable future, and I expect there will be annoying bugs and limited support until the runtime hits v1, so this should be considered an extremely dumb thing to do in a production environment.

[jayair]

Thanks for taking a crack at it!