Validate The Body Of The Blocks Endpoint

For most requests from frontend to DCR we validate the body of the request against a schema to make sure it has the correct shape. For example, for article requests:

https://github.com/guardian/dotcom-rendering/blob/89015504a3457c41d5acda25c754d1e95f9a5366/dotcom-rendering/src/lib/article.ts#L36

The exception is blocks requests, which we type cast instead:

https://github.com/guardian/dotcom-rendering/blob/89015504a3457c41d5acda25c754d1e95f9a5366/dotcom-rendering/src/server/handler.article.web.ts#L69-L70

We should add validation for these requests too.

@DanielCliftonGuardian began this work with #7856, which adds validation specifically for the `blocks` field in the body of these requests.