Bug 1748719 - Make nsStringInputStream more flexible as to the backing data buffer, r=mccr8

This change aims to make the way that the nsStringInputStream owns the backing
data buffer more flexible. This has a few impacts, including allowing
arbitrarily large payload sizes on 64-bit platforms, not requiring as complex
checks around borrowed string buffers or nsTArray data storage, and supporting
custom data ownership schemes such as those used by blobs.

The new system uses a separate refcounted object internally to provide the
contiguous backing buffer, with the nsStringInputStream wrapping it and
providing the `nsIInputStream` interface and cursor. This also avoids issues
around the buffer being mutated during reads, as mutating the `nsIInputStream`
no longer mutates the actual data storage object.

Differential Revision: https://phabricator.services.mozilla.com/D135162