Add Flan-T5 XL truss

[spal1]

This PR adds a Flan-T5 XL truss to the examples/ directory.

You can test this truss by running the following:

import truss

flan_t5_xl_truss = truss.load("flan-t5-xl")

example_input = flan_t5_xl_truss.example("addition example").input
response = flan_t5_xl_truss.docker_predict(example_input)

[joostinyi]

is it possible to add some safe accesses on the secrets object so it's truly optional? – would be great if this example worked for both cases – edit: just ran your test, the key is picked up since it's declared in the config – looks great!