Implement the evidence request mechanism

relates to #19 and #17

set up a test where you install the eGov app (containing the eGovId app) in the wallet.

having that, implement an Evidence request and handler that can, given the path to the evidence, is able to unlock the dapp if necessary, retrieve the requested evidence, and return it (along with either the sRead for the evidence DSU, of the data + signature + signee for that specific bit of info, ex:

• evidence request for egov.pt.id -> should return the full EGovID object and the ssi for that evidence;
• evidence request for egov.pt.id.name -> should return the name property, the signature for that property and the id of the signer. Topic to discuss: should we wrap this info in a DSU?