Fast Fetch pre-requisite for RTC support

### Description

Hello,

I am looking for supporting RTC as an Adserver which does not support Fast Fetch yet.
In https://github.com/ampproject/amphtml/blob/main/extensions/amp-a4a/rtc-documentation.md RTC is introduced as an optional feature of Fast Fetch but I was wondering what is the link between Fast Fetch and RTC.

Wouldn't it be possible to implement RTC and let our Adserver receive targeting keys from the RTC vendors without supporting Fast Fetch? If so, where can I find information to implement RTC in our Adserver tag? I only found this one for the publishers https://github.com/ampproject/amphtml/blob/main/extensions/amp-a4a/rtc-documentation.md

If Fast Fetch is actually required, would you mind providing information about the "validation service" mentioned in https://github.com/ampproject/amphtml/blob/main/ads/google/a4a/docs/Network-Impl-Guide.md ?

> The creative must be sent by the ad network to a validation service which verifies that the creative conforms to the AMPHTML ad specification. If the ad conforms, the creative is rewritten by the validation service and the rewritten creative and a cryptographic signature are returned to the ad network.

Thanks!

### Alternatives Considered

Implement RTC without the need of Fast Fetch so that our Adserver can receive targeting keys in the request.

### Additional Context

_No response_

Start with extensions/amp-a4a/rtc-documentation.md and ads/google/a4a/docs/Network-Impl-Guide.md, then trace the referenced Fast Fetch and validation-service flow. Done means documenting whether RTC requires Fast Fetch for an ad server and, if applicable, where the validation-service implementation information is maintained.

Written by the indexing model from the issue text.