move s3 asset storage related fields to teraslice section of config

[busma13]

When the external asset storage feature was implemented, all the fields in the config were put in the terafoundation section, but they are specific to teraslice so should be in that section.

To move them to teraslice we need to move the validation function. Unfortunately it requires access to terafoundation.connectors so the inputs to validator functions will need to allow for the whole config to be passed in.

[godber]

Lets deprecate the asset_storage_* keys in terafoundation. at the same time as adding them to teraslice. That way we're not making a breaking change to the terafoundation.yml. Also make sure to add comments that allow us to find and remove the asset_storage keys that need to be removed in 2.0.