Refactor to make stun resolve abstract

[elct9620]

[!WARNING] The Linux related code not fully move to stun_linux.go or _linux.go files

• rename internal/session to internal/stun
• make resolve flow to
  • Create OS-specific Stun
  • Start writing packet
  • Write stun packet
  • Parse response
• Split stun_linux.go and darwin_linux.go