Add gradle task to unzip amalgamations to local build directory

[05nelsonm]

current build process is that of:

• build sqlite-jdbc
• amalgamations are then copied to library/driver/sqlite3mc
• cklib compiles natively

This will blow up version control. Need to setup a gradle task to download the amalgamations to root project build directory, which then get unzipped to the project module's build directory at build time so that cklib can compile things.

[05nelsonm]

Everything that is checked into git (jar files) currently should also be compressed so that, at build time, it can be decompressed to the build directory as needed.

[05nelsonm]

Thought about this further. The flow should be this:

• build sqlite-jdbc via external/task.sh
• sign macOS/mingw libs
• zip the jarfile, signed libs, and a SQLite3MultipleCiphers file containing the URL to download the amalgamations.
• commit zip file to separate branch (e.g. sqlite-jdbc-{version})
• master branch downloads the zip file to root project build directory
• unzip jar file and signed libs to library/driver/build so repack task can run
• download amalgamations to root project build directory
• unzip amalgamations to library/driver/build so cklib can build Darwin targets.

[05nelsonm]

Removed all of the unnecessary native libs from library/android-unit-test/src/main/resources/org/sqlitenative, library/driver/src/androidMain/jniLibs, and external/libs/unsigned and added .gitignore files in #110.

It's better, but still have that .jar file and libs from external/libs/signed to figure out.