Support delta updates

[lberrymage]

Although split APKs and compression greatly reduce download sizes, we can reduce them further by caching previous app downloads on the client and generating deltas to patch what's already downloaded. It doesn't really matter what format the deltas take as long as it can be feasibility implemented in both the client and server codebases. Ideally we should use a popular existing format with well-supported libraries for delta generation in Go (for the developer portal) and delta application in Kotlin/Java (for the client). However, since this is a pretty niche need I'm not opposed to writing/translating/maintaining a new delta generator/patcher for an existing format if need be or maybe using C bindings. Chromium's fork of bsdiff looks like an interesting candidate.

Considering how frequently Accrescent checks for updates and that it performs them without user interaction, we probably only want to generate deltas between the last release and the current release.

[lberrymage]

On second thought we might not need to cache previous downloads to patch against them. https://developer.android.com/reference/android/content/pm/ApplicationInfo#sourceDir and https://developer.android.com/reference/android/content/pm/ApplicationInfo#splitSourceDirs allow us to extract installed APKs as needed.

[lberrymage]

Here are some stats for bsdiff:

Sample patch sizes for Accrescent:

0.1.0_0.2.0.patch: 1078153 bytes
0.2.0_0.2.1.patch: 10343 bytes
0.2.1_0.2.2.patch: 427772 bytes
0.2.2_0.3.0.patch: 727669 bytes
0.3.0_0.3.1.patch: 1095496 bytes
0.3.1_0.4.0.patch: 802749 bytes

Avg. patch size: 690364 bytes
Avg. size savings compared to downloading next full update: 3454474 bytes (83.6% cut)

For an example with larger sizes:

Sample patch sizes for Organic Maps:

22021117_22021629.patch: 24134508 bytes
22021629_22021901.patch: 23853112 bytes
22021901_22032304.patch: 31934433 bytes
22032304_22042702.patch: 25466343 bytes
22042702_22052402.patch: 25233031 bytes
22052402_22053110.patch: 25239447 bytes

Avg. patch size: 25976812 bytes
Avg. size savings compared to downloading next full update: 33171765 bytes (56.1% cut)

These numbers aren't comparable across apps for obvious reasons, but they should give a rough idea of how useful this feature would really be.

[lberrymage]

Searching suggests the Play Store uses bsdiff for delta updates.

[lberrymage]

bsdiff is simple enough that we can feasibly rewrite it in Rust. A naive translation of https://github.com/mendsley/bsdiff is acceptable as an MVP but it may be desirable to implement the bsdiff 6 improvements outlined in https://www.daemonology.net/papers/thesis.pdf.

[lberrymage]

Note that only the bsdiff code itself, that is, the suffix sorting and patch generation logic, is feasible for us to re-implement in Rust. We will likely be forced to use C bindings to bzip2 for the compression portion of the algorithm, in which case delta patching should be done within an isolatedProcess, adhering to the rule of 2.

[lberrymage]

We most likely want to use zstd instead of bzip2 since bzip2 is largely unmaintained.