Automatic Asset URL Signing

[alukach]

Given that a) the Auth Proxy service is capable of validating incoming requests via its Auth Check feature; b) the Auth Proxy service is proxying responses from the upstream STAC API to end-users; and c) URL signing is commonly an inexpensive process; it would be achievable to automatically parse the response bodies returned from the upstream STAC API and replace the asset URLs with signed URLs, thereby eliminating the signing step described in the STAC Authentication Extension.

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blocked by #12