Implement the mediator out of band invitation

[hugoib]

OOB Inv: Is an unencrypted message (In the form of an URL or QR code that contains a b64urlencode JWM) with the mediator public DID.

This is the first step in the interaction with the Mediator. The following one is the mediation coordination where a 'request mediation' request is created and performed.

Examples:

The parsed embedded query parameter looks like:

Documentation:

• Out Of Band Messages
• Aries RFC 0434: Out-of-Band Protocol 1.1
• Contact Exchange

[Christiantyemele]

This issue was merged here

[francis-pouatcha]

https://github.com/adorsys/didcomm-mediator-rs/tree/iss-67--oob-invitation, https://github.com/adorsys/didcomm-mediator-rs/pull/74