Update P4Runtime and use FetchContent instead of a submodule.

.gitmodules

@@ -2,9 +2,6 @@
 	path = test/frameworks/gtest
 	url = https://github.com/google/googletest.git
 	ignored = untracked
-[submodule "control-plane/p4runtime"]
-	path = control-plane/p4runtime
-	url = https://github.com/p4lang/p4runtime.git
 [submodule "backends/ebpf/runtime/contrib/libbpf"]
 	path = backends/ebpf/runtime/contrib/libbpf
 	url = https://github.com/libbpf/libbpf

control-plane/CMakeLists.txt

@@ -12,9 +12,25 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+
+# Print download state while setting up P4Runtime.
+set(FETCHCONTENT_QUIET_PREV ${FETCHCONTENT_QUIET})
+set(FETCHCONTENT_QUIET OFF)
+# Fetch and declare the P4Runtime library.
+FetchContent_Declare(
+  p4runtime
+  GIT_REPOSITORY https://github.com/p4lang/p4runtime.git
+  GIT_TAG        d76a3640a223f47a43dc34e5565b72e43796ba57
+  GIT_PROGRESS TRUE
+  GIT_SHALLOW TRUE
+)
+FetchContent_MakeAvailable(p4runtime)
+set(FETCHCONTENT_QUIET ${FETCHCONTENT_QUIET_PREV})
+message("Done with setting up P4Runtime for P4C.")
+
 # The standard P4Runtime protocol buffers message definitions live in the PI
 # repo, which is included in this repo as a submodule.
-set (P4RUNTIME_STD_DIR ${CMAKE_CURRENT_SOURCE_DIR}/p4runtime/proto)
+set (P4RUNTIME_STD_DIR ${p4runtime_SOURCE_DIR}/proto)
 set (P4RUNTIME_INFO_PROTO ${P4RUNTIME_STD_DIR}/p4/config/v1/p4info.proto)
 set (P4RUNTIME_INFO_GEN_SRCS ${CMAKE_CURRENT_BINARY_DIR}/p4/config/v1/p4info.pb.cc)
 set (P4RUNTIME_INFO_GEN_HDRS ${CMAKE_CURRENT_BINARY_DIR}/p4/config/v1/p4info.pb.h)
@@ -58,6 +74,7 @@ add_custom_command(OUTPUT ${P4RUNTIME_GEN_SRCS} ${P4RUNTIME_GEN_HDRS}
           -I ${P4RUNTIME_STD_DIR} -I${CMAKE_CURRENT_SOURCE_DIR}
           --cpp_out ${CMAKE_CURRENT_BINARY_DIR}
           --python_out ${CMAKE_CURRENT_BINARY_DIR}
+          --experimental_allow_proto3_optional
           ${P4RUNTIME_PROTO}
   DEPENDS ${P4RUNTIME_PROTO}
   COMMENT "Generating protobuf files"