Add Gradle plugin for building libraries

[thunderbiscuit]

Description

This PR adds two internal Gradle plugins which live in the buildSrc directory. The purpose of the plugins is to replace the build.sh script by a more idiomatic and customizable workflow using Gradle tasks.

Notes to the reviewers

By default the :android:buildAndroidLib task will build all versions of bdk-android, which currently are the arm64-v8a (for mobile devices) and the x86_64 (for emulators).

Users can opt out of a particular build if they'd like by excluding the task like so:

./gradlew :android:buildAndroidLib --exclude-task buildAndroidX86_64Binary

This PR is open as a discussion piece on the direction I am taking for implementing the next iteration of our build logic.

To test the plugins, first see that the tasks have been added using ./gradlew tasks, which should add the Bitcoindevkit group to the list of Gradle tasks:

Then run either tasks to build the given library:

./gradlew :jvm:buildJvmLib
./gradlew :android:buildAndroidLib

All Submissions:

• I've signed all my commits

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[thunderbiscuit]

The plugin adds a number of sub-tasks, but only the aggregate buildJvmLib and buildAndroidLib should be run by the end user.

[notmandatory]

Looks like this needs a rebase.

[notmandatory]

This is a great improvement, much cleaner to use gradle than build.sh. I tested building and running the jvm and android tests and works great on my linux box (after making the little requested changes). A rebase it also needed I believe. I also think it's ok to remove the build.sh in this PR.

[thunderbiscuit]

Good idea to remove the build shell script. It forces a change in the CI as well but I think it should work. Fixed in latest commit.

[thunderbiscuit]

I tried resolving the conflicts right into the Github UI instead of rebasing (it offered me the option to do so, which I'd never seen before... first time trying this). Anyway it looks like it worked nicely.

[thunderbiscuit]

This PR implicitly removes support for the i686 architecture for bdk-android. See discussion in #52.

[notmandatory]

ACK a9f42dd

Thanks for the fixes! I think this is ready to go let's chat about when to merge.