Add a tool to merge two partial OTAs

Some partners have large number of products that share common
images. Generating OTAs for all these products waste a lot of CPU
cycles, because we waste time diffing the same images multiple times.
To mitigate this, add a tool for merging partial OTAs. Partners can
generate a partial OTA for common images, and re-use the generated OTA
by merging with each product specific OTA.

Bug: 227848550
Test: Run merge_otas.py on 4 OTAs, install the generated OTA

Change-Id: Ie38d522cdc9b89b1123b3d8190516c3648f99788