DeRecIdentity: Change type of `contact` and `address` variables to `String`

[diptimahamuni]

Problem

The variables contact and address are currently of type URI. This makes the variables less versatile in terms of their usage. For example, URI variables could not be embedded easily in a QR code. Changing the variables to type String will make usage of them simpler.

Solution

In class DeRecIdentity, the type of contact and address variables should be updated to String as follows:

private final String contact;
private final String address;

Getters and setters for these variables should be changed accordingly as well.

[diptimahamuni]

If this change is made, the transportURI in DeRecContact should be updated to type String as well (See #61).