webrtc: support `FIN_ACK`

[mxinden]

Description

Implement webrtc FIN_ACK specification.

libp2p/specs#582

Motivation

Closing data channels without data loss.

Current Implementation

Are you planning to do it yourself in a pull request?

No

[elijahhampton]

@mxinden Still looking for help on this one? I can take it

[dariusc93]

@mxinden Still looking for help on this one? I can take it

Hey! You're welcome to take a shot at it as any PRs are welcomed :)

[tesol2y090]

Hi, I can take this issues as part of PLDG cohort 1.