support for multiple supergraph contracts

[Geal]

in some use cases we might need to choose which supergraph (and subgraphs, etc) is used depending on a condition in the request (hostname, HTTP header, etc) and the Apollo Studio Contracts feature is the most motivating reason to support this as today it requires running multiple routers.

keywords: dynamic, schema

[abernix]

Could be a good candidate for a Tower layer!

[flymedllva]

@abernix Any thoughts on this? We want to choose a scheme based on the version specified in the headers

[smyrick]

Just commenting to add some SEO terms so we don't create multiple issues:

Multiple supergraphs, many contracts, dynamic supergraph fields, dynamic contracts, per request supergraph