Add support for building libsignal from source

gradle.properties

@@ -0,0 +1,2 @@
+# Uncomment this to build libsignal from source.
+# libsignalClientPath=../libsignal

settings.gradle.kts

@@ -0,0 +1,10 @@
+// To build libsignal from source, set the libsignalClientPath property in gradle.properties.
+val libsignalClientPath = if (extra.has("libsignalClientPath")) extra.get("libsignalClientPath") else null
+if (libsignalClientPath is String) {
+  includeBuild(rootDir.resolve(libsignalClientPath + "/java")) {
+    name = "libsignal-client"
+    dependencySubstitution {
+      substitute(module("org.signal:libsignal-client")).using(project(":client"))
+    }
+  }
+}