Refactor metadata, specifically move required_pkgs

[juliasilge]

We've decided to keep as much metadata as possible outside of the binary bundle, stored in plain text (YAML). As of today, here is what is wrapped up into the binary object:

vetiver-r/R/pin-read-write.R

Lines 38 to 40 in 8875142

	x = list(model = vetiver_model$model,
	ptype = vetiver_model$ptype,
	required_pkgs = vetiver_model$metadata$required_pkgs),

In R, we don't have any tool like pydantic that can completely/accurately store that can store the signature/schema/ptype of a dataframe in plain text like JSON or YAML. This means that for now, we will keep the prototype / ptype in the list() that gets serialized to binary when storing.

It is possible/easier to move required_pkgs to the user metadata slot, since it's just plain text.

[juliasilge]

Related to #116