[proposal] Add Trust Ping Protocol

[nickreynolds]

Is your feature request related to a problem? Please describe.
Implement the functionality of the Trust Ping Protocol, as defined here: https://identity.foundation/didcomm-messaging/spec/#trust-ping-protocol-10

Describe the solution you'd like
A helper function for sending Trust Pings (function should accept recipient DID, and handle packing and sending of correctly defined DIDCommMessage) as well as a message handler for processing Trust Pings (and sending Trust Ping Responses, when required, which is the standard flow)

Describe alternatives you've considered
Work in progress here: https://github.com/uport-project/veramo/tree/nickreynolds/trust-ping

Additional context
Implementation from Aviary Tech here: https://github.com/aviarytech/didcomm/blob/master/src/lib/protocols/trust-ping/2.0/ping.ts