Oob builder for messages crate

[Patrik-Stas]

Building OOB invitation is fundamental way for anyone to get started with aries-vcx, in its current state. The first thing you ever need if you want to simulate a basic use case with aries-vcx is to create the oob invitation.
This need came up for example in:

• Create Alice Faber CLI Demo for aries_vcx (Rust) #870
• mentorship mediator project https://github.com/nain-F49FF806/axum-test-server/tree/main/src

Currently in aries_vcx/src/handlers/out_of_band there's OutOfBandSender which is position on same level like other protocol handlers, but in fact, this is just a builder for OOB messages.
More-over, working with the OutOfBandSender is currently difficult, because it's method

pub fn append_service(mut self, service: &OobService) -> Self {

ultimately requires you to import diddoc_legacy into your project, which is not obvious.

Proposal:

• There's already established consensus that builder for messages crate is necessary, and constructing message by hand is currently cumbersome.
• So let's use this as opportunity to create first message builder, specifically for oob invites; also get rid of OutOfBandSender in the process

Additional note:

There's also OutOfBandReceiver - we'll get rid of that in separate step - it contains method. nonmediated_connection_exists which is being used in some NodeJS tests.

[Patrik-Stas]

We now have builders for Messages crate, but we should yet do something about OutOfBandSender and OutOfBandReceiver