Include Bazel Genrule for Fast SSZ (#5070)

* rem slasher proto
* Merge branch 'master' of github.com:prysmaticlabs/prysm
* Merge branch 'master' of github.com:prysmaticlabs/prysm
* Merge branch 'master' of github.com:prysmaticlabs/prysm
* Merge branch 'master' of github.com:prysmaticlabs/prysm
* Merge branch 'master' of github.com:prysmaticlabs/prysm
* Merge branch 'master' of github.com:prysmaticlabs/prysm
* Merge branch 'master' of github.com:prysmaticlabs/prysm
* Merge branch 'master' of github.com:prysmaticlabs/prysm
* Merge branch 'master' of github.com:prysmaticlabs/prysm
* Merge branch 'master' of github.com:prysmaticlabs/prysm
* Merge branch 'master' of github.com:prysmaticlabs/prysm
* included new ssz bzl rule
* Merge branch 'master' into add-in-starlark-rule
* Update tools/ssz.bzl

Co-Authored-By: Preston Van Loon <preston@prysmaticlabs.com>

tools/ssz.bzl

@@ -0,0 +1,70 @@
+load(
+    "@io_bazel_rules_go//go:def.bzl",
+    "GoLibrary",
+)
+
+def _ssz_go_proto_library_impl(ctx):
+    go_proto = ctx.attr.go_proto
+
+    generated_pb_go_files = go_proto[OutputGroupInfo].go_generated_srcs
+
+    # Run the tool on the generated files
+    package_path = generated_pb_go_files.to_list()[0].dirname
+    output = ctx.outputs.out
+    args = [
+        "--output=%s" % output.path,
+        "--path=%s" % package_path,
+    ]
+    if len(ctx.attr.objs) > 0:
+        args += ["--objs=%s" % ",".join(ctx.attr.objs)]
+    ctx.actions.run(
+        executable = ctx.executable.sszgen,
+        progress_message = "Generating ssz marshal and unmarshal functions",
+        inputs = generated_pb_go_files,
+        arguments = args,
+        outputs = [output],
+    )
+
+"""
+A rule that uses the generated pb.go files from a go_proto_library target to generate SSZ marshal
+and unmarshal functions as pointer receivers on the specified objects. To use this rule, provide a 
+go_proto_library target and specify the structs to generate methods in the "objs" field. Lastly, 
+include your new target as a source for the go_library that embeds the go_proto_library.
+
+Example:
+go_proto_library(
+  name = "example_go_proto",
+   ...
+) 
+
+ssz_gen_marshal(
+  name = "ssz_generated_sources",
+  go_proto = ":example_go_proto",
+  objs = [ # omit this field to generate for all structs in the package.
+    "AddressBook",
+    "Person",
+  ],
+)
+
+go_library(
+  name = "go_default_library",
+  srcs = [":ssz_generated_sources"],
+  embed = [":example_go_proto"],
+  deps = SSZ_DEPS,
+)
+"""
+ssz_gen_marshal = rule(
+    implementation = _ssz_go_proto_library_impl,
+    attrs = {
+        "go_proto": attr.label(providers = [GoLibrary]),
+        "sszgen": attr.label(
+            default = Label("@com_github_ferranbt_fastssz//sszgen:sszgen"),
+            executable = True,
+            cfg = "host",
+        ),
+        "objs": attr.string_list(),
+    },
+    outputs = {"out": "generated.ssz.go"},
+)
+
+SSZ_DEPS = ["@com_github_ferranbt_fastssz//:go_default_library"]