Indicate bento supported versions within ExtensionSpec

[honeybadgerdontcare]

Description

The AMP-WP plugin using the Optimizer auto imports extension script tags and needs assistance in knowing which version to include; original or Bento.

To assist in this, the ExtensionSpec will have a new field bento_supported_version which will list the versions that are usable for Bento.

Alternatives Considered

bundles.config.extensions.json contains some metadata but it is not sufficient. It is possible to use requires and satisfies logic from the protoascii but that is overly complex and may be error prone from other uses of requires and satisfies.

Additional Context

The issue came to a head when amp-facebook 1.0 was introduced with a different requires_extension. This allows for amp-facebook-comments to be included as a tag and determining which script extension tag to include is not immediately apparent (0.1 or 1.0). More background see comments starting here.

@westonruter @caroqliu