Feature/ack credential

[jakubkoci]

This implements the ack part of the issue #64. I suggest closing that issue after the merge of this PR and eventually create other issues for the remaining functionality we would like to implement related to credential exchange.

What I see as remaining:

• full implementation of message threading with attributes like sender_order and received_orders
• alternative beginning with a credential proposal

[jakubkoci]

It should also fix #117 if I didn't forget something :)

[TimoGlastra]

Awesome! I've added a lot of comments, but most are small, nitpicks or opinionated

[TimoGlastra]

I think if all values are optional, options should be optional. This way you wouldn't have to pass an empty object to the constructor.

Suggested change

	public constructor(options: CredentialAckMessageOptions) {
	public constructor(options?: CredentialAckMessageOptions) {

[jakubkoci]

That's a good point and I agree. However, if I do that we would need to change the logic to something like this:

    if (options) {
      this.id = options.id ?? this.generateId();
    } else {
      this.id = this.generateId();
    }

And I don't like that. I suggest to keep it for now and maybe change later if we see other cases like this one. Just to keep it consistent, you know.

[TimoGlastra]

Good point. We definitely don't want that.

Ideally we should get rid of the if (options) in all classes. There are others that have the same problem. class-transformer calls new XXX while it should use reflection directly. See the issue (and my comment). I'll try to take a look as it will be of huge benefit to us.