Fix AuthenticationExtensionsAuthenticatorInputs/Outputs CDDL

According to the CDDL grammar, after a control operator (called `ctlop` in the
ABNF grammar), there can only be a `type2` production:
https://datatracker.ietf.org/doc/html/rfc8610#appendix-B

In a `type2` production, wrapping parentheses can only be used to wrap a `type`
production. `tstr => any` is a `group` production, and needs to be wrapped in
curly braces or brackets.

In other words, from a CDDL grammar perspective, this is an invalid type:
  `foo .within ( tstr => any )`

This is valid:
  `foo .within { tstr => any }`

This update fixes the CDDL type definitions that used the `.within` operator
with an invalid type2.

index.bs

@@ -4013,7 +4013,7 @@ This is a dictionary containing the [=client extension output=] values for zero
 
 ```
 AuthenticationExtensionsAuthenticatorInputs = {
-  * $$extensionInput .within ( tstr => any )
+  * $$extensionInput .within { tstr => any }
 }
 ```
 
@@ -4028,7 +4028,7 @@ This type is not exposed to the [=[RP]=], but is used by the [=client=] and [=au
 
 ```
 AuthenticationExtensionsAuthenticatorOutputs = {
-  * $$extensionOutput .within ( tstr => any )
+  * $$extensionOutput .within { tstr => any }
 }
 ```