reference license, template, system as files

[mxyng]

this change allows certain layers to take files as values. the final value for the layer is the content of the file

FROM llama3
LICENSE ./meta-llama/Meta-Llama-3-8B-Instruct/LICENSE
LICENSE ./meta-llama/Meta-Llama-3-8B-Instruct/USE_POLICY.md

any value that does not reference a file will use the original value

FROM foo
TEMPLATE {{ .System }} {{ .Prompt }}

the file value does not support wildcards or globs