feat: add credential_offer_uri as default

[coplat]

Description of change

Updates CreateCredentialOffer handler to default offer creations to by reference as opposed to by value.

Links to any relevant issues

Fixes issue #165

How the change has been tested

cargo test, some tests still failing - being worked on.

Definition of Done checklist

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• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
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• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
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[nanderstabel]

Makes total sense that you the same method to construct the form_url_encoded_credential_offer here as well! However, seeing this makes me think that 'recreating' the form_url_encoded_credential_offer here was not really a good idea in the first place.

We should probably discuss this further in a call, but my idea improve this is as follows:

1. We get rid of the CreateFormUrlEncodedCredentialOffer command
2. We keep the FormUrlEncodedCredentialOfferCreated event the way you updated it, but we add it to the CreateCredentialOffer command. So CreateCredentialOffer will result in two events: CredentialOfferCreated and FormUrlEncodedCredentialOfferCreated.
3. In SendCredentialOffer we can simply assume that self.form_url_encoded_credential_offer contains the form_url_encoded_credential_offer string.

[nanderstabel]

A bunch of tests related to IotaRms are currently failing because of the discontinuation of the did:iota:rms DID Method (see #167) so this is unrelated to your PR. I will fix that problem in a separate PR so this problem will resolve itself eventually 👍

[nanderstabel]

More details for updating the Postman Collection:

Current test script flow in the Postman Collection (Credential Offer by Value)

1. POST Issuance/w3c_vc_credentials
2. POST Issuance/offers

• Its Tests script extracts the pre-authorized_code from the Response and stores it in the PRE_AUTHORIZED_CODE environment variable.

3. GET oid4vci/oauth-authorization-server, its Tests script:

• Stores the issuer in the ISSUER environment variable.
• Stores the token_endpoint in the TOKEN_ENDPOINT environment variable.

4. GET oid4vci/openid-credential-issuer
5. POST oid4vci/token (uses the PRE_AUTHORIZED_CODE environment variable)
   • Its Tests script extracts the access_token from the Response and stores it in the ACCESS_TOKEN environment variable.
6. POST oid4vci/credential (uses the ACCESS_TOKEN environment variable)

Desired test script flow:

1. Same as above
2. The test script should extract (and decode) the value for credential_offer_uri and strore it in a new environment variable called CREDENTIAL_OFFER_URI.
3. A new request should be added: GET oid4vci/Get Credential Offer by Value

• Its URL should be {{CREDENTIAL_OFFER_URI}}
• Its Tests script should extract the pre-authorized_code from the Response and store it in the PRE_AUTHORIZED_CODE environment variable (note that the response is not a URL encoded string now, but alrady a simple JSON object)

4. Same as step 3 above
5. Same as step 4 above
6. Same as step 5 above
7. Same as step 6 above

---

• The new CREDENTIAL_OFFER_URI environment variable should be added first to the ssi-agent Collection (in the 'Variables' tab)
• So that means that only step 2 needs to be updated, and a new request with Test script should be added.