OPDS Bookserver should support BasicAuth Lending + Web-view link to borrow

## Web-view

Aldiko reader uses the web-view approach where clicking borrow link sends the user offsite to login to a partner service + to fulfill the book.

Here's an example @hadriengardeur put together for the "web-view" link case:
http://feedbooks.github.io/opds-test-catalog/demo/openlibrary.atom

`http://opds-spec.org/acquisition/borrow` is used to specify a acquisition/borrow link (which sends the user to openlibrary [or alternativelyIA] to login + fulfill)
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## Basic Auth

Alternatively, SimplyE has the user enter their credentials when they first select a source library. These credentials are stored in app and then passed along with borrow requests to a different acquisition link (?)
https://docs.google.com/document/d/1-_0HHt664bDjybtCauBJXUSDXiT-Clg1sZUVNxHyLjw/edit#heading=h.vr02rhcxbns3

@bfalling @hornc