Update to attestedsvc map

[achamayou]

This PR contains the following changes:

1. did:attestedsvc: issuer payloads go from:

{
   1: -35,                ; alg: ES384
   2: [ "msft-css-dev" ],  ; crit
   4: bstr,               ; kid: SHA256 hash of canonical COSE_Key
  15: { ... },            ; CWT-Claims including issuer/subject
  "msft-css-dev": { ... } ; attestation map 
}

To:

{
   1: -35,                ; alg: ES384
   2: [ "attestedsvc" ], ; crit
   4: bstr,               ; kid: SHA256 hash of canonical COSE_Key
  15: { ... },            ; CWT-Claims including issuer/subject
  "attestedsvc": { "svc_id": "msft-css-dev", ... } ; attestation map as before
}

2. Authentication enforces that iss starts with did:attestedsvc:{svc_id}:
3. Authentication enforces that crit contains and only contains "attestedsvc"

@briankr-ms this is svc_id rather than svc-id for consistency with the other fields in the map (uvm_endorsement etc).

Closes #321

[ivarprudnikov]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).