[Content Addressable] Allow `gem push` to accept content addressable selectors and resolve matching file

[OughtPuts]

gem push accepts --platform and --ruby selectors and resolves the matching file by reading each file's internal spec.

[OughtPuts]

WIP branch master...Shopify:rubygems:ho/ca-changes-gem-push

[OughtPuts]

Depends on gem build

[OughtPuts]

@girachawda #1

[OughtPuts]

(All rubygems work should branch off the feature branch in the shopify fork)
https://github.com/Shopify/rubygems/tree/feature-branch-ca-changes-rubygems