add example for signed request to IAR endpoint

[jogu]

It's possibly odd that in an signed request this would mean:

"interaction_types_supported": "openid4vp_presentation,redirect_to_web"

when possibly people might expect:

"interaction_types_supported": [ "openid4vp_presentation", "redirect_to_web"]

(Not sure I'm suggesting changing anything, but an example of the contents of a signed request could be useful to avoid misinterpretation.)

Originally posted by @jogu in #509 (comment)