[exporterhelper] Switch request context encoding from map to KV list (#13216)

Resolves
#13188 (review)

This also required moving to gogoproto to reuse existing
opentelemetry.proto.common.v1.KeyValue message.

Benchmarks:
```
goos: darwin
goarch: arm64
pkg: go.opentelemetry.io/collector/pdata/xpdata/request
cpu: Apple M1 Max

before:
BenchmarkEncodeDecodeContext-10    	  519790	      2278 ns/op	    1376 B/op	      22 allocs/op

after:
BenchmarkEncodeDecodeContext-10    	  747158	      1600 ns/op	     661 B/op	      18 allocs/op
```

Author: dmitryax

Makefile

@@ -252,13 +252,13 @@ genpdata:
 
 INTERNAL_PROTO_SRC_DIRS := exporter/exporterhelper/internal/queue pdata/xpdata/request/internal
 INTERNAL_PROTO_FILES := $(foreach dir,$(INTERNAL_PROTO_SRC_DIRS),$(wildcard $(dir)/*.proto))
-INTERNAL_PROTOC := $(DOCKERCMD) run --rm -u ${shell id -u} -v${PWD}:${PWD} -w${PWD} ${DOCKER_PROTOBUF} --proto_path=${PWD} -I/usr/include/github.com/gogo/protobuf -I${PWD}/$(PROTO_INTERMEDIATE_DIR) --go_out=${PWD}
+INTERNAL_PROTOC := $(DOCKERCMD) run --rm -u ${shell id -u} -v${PWD}:${PWD} -w${PWD} ${DOCKER_PROTOBUF} --proto_path=${PWD} -I/usr/include/github.com/gogo/protobuf -I${PWD}/$(PROTO_INTERMEDIATE_DIR) --gogofaster_out=plugins=grpc,paths=source_relative:.
 
 .PHONY: genproto_internal
 genproto_internal:
 	@echo "Generating Go code for internal proto files"
 	@echo "Found proto files: $(INTERNAL_PROTO_FILES)"
-	$(foreach file,$(INTERNAL_PROTO_FILES),$(call exec-command,$(INTERNAL_PROTOC) --go_opt=paths=source_relative $(file)))
+	$(foreach file,$(INTERNAL_PROTO_FILES),$(call exec-command,$(INTERNAL_PROTOC) $(file)))
 
 ALL_MOD_PATHS := "" $(ALL_MODULES:.%=%)