Rename Go module to allow installing with go install

[ben-krieger]

Goal: Install rpc-go with go install github.com/open-amt-cloud-toolkit/rpc-go/v2/cmd/rpc@latest.

Steps taken:

• Modify module name in go.mod to github.com/open-amt-cloud-toolkit/rpc-go/v2 to reflect location and proper versioning (required by Go toolchain)
• Move contents of cmd to cmd/rpc for proper executable naming
• Update self-imports with find . -name '*.go' -exec sed -i 's|"rpc/|"github.com/open-amt-cloud-toolkit/rpc-go/v2/|' '{}' \;
• Fix import grouping with find . -name '*.go' -exec goimports -local github.com/open-amt-cloud-toolkit -w '{}' \;
• Ensure proper formatting with find . -name '*.go' -exec gofmt -s -w '{}' \;
• Update README and Makefile with new cmd/rpc path