Add support for multiple 'content collections'

[kpfleming]

While considering using this tool for publishing a blog or a book, we'd like to be able to have 'previews' of modified versions of the content (for example, in a GitHub Pull Request), without having to create a temporary Compute service to hold the preview collection.

Ideas for how this might be done:

• Use the existing .publish-id as a 'collection ID'
• Allow a query parameter on an incoming request to provide an alternative collection ID
• When an alternative collection ID has been provided, use it to obtain the contentAssetMetadataMap from the KV Store, instead of using the static one in the service
• Improve the publish process to upload assets to the KV store and to upload the contentAssetMetadataMap (using a key derived from the collection ID) - this would also need Add ability to opt-out of writing metadata to fastly.toml #28 to avoid updating fastly.toml

[harmony7]

This feature has been released as part of v7. #37