Add plugin in buildSrc to provide some build metadata to app at runtime

build.gradle.kts

buildSrc/build.gradle

...roject/briar/desktop/builddata/AbstractBuildDataTask.java

...briarproject/briar/desktop/builddata/BuildDataPlugin.java

...ect/briar/desktop/builddata/BuildDataPluginExtension.java

...org/briarproject/briar/desktop/builddata/FileBuilder.java

.../briar/desktop/builddata/GenerateBuildDataSourceTask.java

...n/java/org/briarproject/briar/desktop/builddata/Util.java

...project.briar.desktop.build-data-gradle-plugin.properties

src/main/kotlin/org/briarproject/briar/desktop/Main.kt