add flag to allow putting out binary whenever (#312)

I'd like to build Rust code like this into `wasm32-unknown-unknown`. 
https://github.com/Shikugawa/envoy-wasm-rust-playground/blob/master/example/src/lib.rs

This code is build with this build config. 
https://github.com/Shikugawa/envoy-wasm-rust-playground/blob/master/example/Cargo.toml

We want to put out binary but not want to build with `--cargo-type=lib` but `--cargo-type=cdylib`. In the current implementation, we can't put out binary without using `--cargo-type=bin`. This was a critical problem for us so that patched this.

I'm considering to use this like this. This will put out `test_rust.wasm` with this command.

```
bazel build //:test_rust --platforms=@io_bazel_rules_rust//rust/platform:wasm
```

```
rust_binary(
    name = "test_rust",
    srcs = ["test_rust.rs"],
    crate_type = "cdylib",
    out_binary = True,
)
```

Author: Shikugawa

rust/private/rust.bzl

@@ -148,12 +148,14 @@ def _rust_binary_impl(ctx):
     else:
         output = ctx.actions.declare_file(ctx.label.name)
 
+    crate_type = getattr(ctx.attr, "crate_type")
+
     return rustc_compile_action(
         ctx = ctx,
         toolchain = toolchain,
         crate_info = CrateInfo(
             name = crate_name,
-            type = "bin",
+            type = crate_type,
             root = _crate_root_src(ctx, "main.rs"),
             srcs = ctx.files.srcs,
             deps = ctx.attr.deps,
@@ -454,6 +456,10 @@ _rust_binary_attrs = {
         cfg = "host",
         allow_single_file = True,
     ),
+    "crate_type": attr.string(
+        default = "bin",
+    ),
+    "out_binary": attr.bool(),
 }
 
 rust_binary = rule(

rust/private/rustc.bzl

@@ -373,6 +373,10 @@ def rustc_compile_action(
         files = dep_info.transitive_dylibs.to_list() + getattr(ctx.files, "data", []),
         collect_data = True,
     )
+    
+    out_binary = False
+    if hasattr(ctx.attr, "out_binary"):
+        out_binary = getattr(ctx.attr, "out_binary")
 
     return [
         crate_info,
@@ -381,7 +385,7 @@ def rustc_compile_action(
             # nb. This field is required for cc_library to depend on our output.
             files = depset([crate_info.output]),
             runfiles = runfiles,
-            executable = crate_info.output if crate_info.type == "bin" else None,
+            executable = crate_info.output if crate_info.type == "bin" or out_binary else None,
         ),
     ]