Add support for remote-execution / caching of turbine actions

Test: Ran a sample turbine action with:
RBE_TURBINE_EXEC_STRATEGY="remote" RBE_TURBINE="true" ... use_rbe m out/soong/.intermediates/external/protobuf/libprotobuf-java-micro/android_common/turbine/libprotobuf-java-micro.jar
and checked in reproxy_log.txt that remote-metadata had status SUCCESS.

Bug: b/156765475
Change-Id: I85c334e6998c6dd2a68460b3580d50fffd900e6f
diff --git a/java/config/config.go b/java/config/config.go
index 1344ceb..fa19afb 100644
--- a/java/config/config.go
+++ b/java/config/config.go
@@ -148,6 +148,7 @@
 	pctx.VariableFunc("REJavacExecStrategy", remoteexec.EnvOverrideFunc("RBE_JAVAC_EXEC_STRATEGY", remoteexec.LocalExecStrategy))
 	pctx.VariableFunc("RED8ExecStrategy", remoteexec.EnvOverrideFunc("RBE_D8_EXEC_STRATEGY", remoteexec.LocalExecStrategy))
 	pctx.VariableFunc("RER8ExecStrategy", remoteexec.EnvOverrideFunc("RBE_R8_EXEC_STRATEGY", remoteexec.LocalExecStrategy))
+	pctx.VariableFunc("RETurbineExecStrategy", remoteexec.EnvOverrideFunc("RBE_TURBINE_EXEC_STRATEGY", remoteexec.LocalExecStrategy))
 
 	pctx.HostJavaToolVariable("JacocoCLIJar", "jacoco-cli.jar")