Add submodules for tier-1 rust backends.

Per https://github.com/mozilla/cubeb/wiki/Backend-Support the rust
backends for pulse and coreaudio are tier-1 and the older C++ backends
are tier-4. So, add these backends to cubeb by default.

.gitmodules

@@ -4,3 +4,11 @@
 [submodule "cmake/sanitizers-cmake"]
 	path = cmake/sanitizers-cmake
 	url = https://github.com/arsenm/sanitizers-cmake
+[submodule "src/cubeb-coreaudio-rs"]
+	path = src/cubeb-coreaudio-rs
+	url = https://github.com/mozilla/cubeb-coreaudio-rs
+	branch = trailblazer
+[submodule "src/cubeb-pulse-rs"]
+	path = src/cubeb-pulse-rs
+	url = https://github.com/mozilla/cubeb-pulse-rs
+	branch = dev

src/cubeb-coreaudio-rs

@@ -0,0 +1 @@
+Subproject commit 2407441a2f67341a0e13b4ba6547555e387c671c

src/cubeb-pulse-rs

@@ -0,0 +1 @@
+Subproject commit 8678dcab1c287de79c4c184ccc2e065bc62b70e2