Credential Offer section improvements

[yaromin]

I am proposing to have another look at the Credential Offer section and consider the following changes to make it more concise and clearer.

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The Credential Issuer passes the Credential Offer to the Wallet, by value or by reference, using HTTP GET request or an HTTP redirect to the Wallet's Credential Offer Endpoint defined in (#client-metadata).

The request must contain a single URI query parameter that is either:

• credential_offer: Object with the Credential Offer parameters, which MUST NOT be present when credential_offer_uri parameter is present; or
• credential_offer_uri: URI using the https scheme that references a resource containing a JSON object with the Credential Offer parameters. This MUST NOT be present when credential_offer parameter is present.

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The motivation behind these changes is following:

• In the original version the Credential Offer has more than one meaning and I would argue that we have a loop where Credential Offer contains a Credential Offer inside,
• passed by reference rather than sent by reference,
• URI is a type by itself, so explanation that is a string is redundant,
• I changed URL to URI for consistency,
• mentioning credential_offer and credential_offer_uri more than once is not needed,
• other improvements for clarity

In addition to that I know that we already discussed how to express mutual exclusivity of these parameters, but please note that the current wording introduces redundancy by jointly using "single parameter", "either ... or", and "MUST NOT be present..." language. The "single parameter" + "either ... or" would be enough and would keep this section easier more concise and easier to understand. I encourage everyone to reconsider this option.

[Sakurann]

@yaromin PR would be appreciated if you have time