Add toml markup for Platform struct

[kiashok]

Addresses issue #1073 to add toml markup for Platform struct

[kiashok]

ping on this PR

[tianon]

(soft) NACK; #1073 (comment) (see also pelletier/go-toml#880)

(Soft because I'm still willing to be convinced, but I think we need more spec maintainer opinions to be expressed for this to really move one way or the other)

[sudo-bmitch]

This gets a decline from me as well unless there's a response to #1073 (comment) showing why it's needed.

[sudo-bmitch]

This was brought up in today's meeting (cleaning up old PRs) and the decision was to defer to the TOML default parsing of the struct, or to have the containerd specific requirements happen outside of the OCI spec. Our use of JSON aligns with the rest of the spec that marshals the data into JSON for the descriptors and config blob content. So without a TOML use case here, it didn't make sense to add into OCI directly.