Add nascent document type support for rpcv2Cbor

As the compatibility of document types across protocols that use different
wire serialization formats (JSON, CBOR) is built, it must be possible to
specify them together. Using documents with rpcv2Cbor is still a DANGER
level event, as it won't work in most cases.

Author: kstich

smithy-protocol-traits/src/main/resources/META-INF/smithy/smithy.protocols.rpcv2.smithy

@@ -18,7 +18,8 @@ use smithy.api#httpError
 @traitValidators(
     "rpcv2Cbor.NoDocuments": {
          selector: "service ~> member :test(> document)"
-         message: "This protocol does not support document types"
+         message: "This protocol does not support document types in most possible scenarios."
+         severity: "DANGER"
     }
 )
 structure rpcv2Cbor {

smithy-protocol-traits/src/test/resources/software/amazon/smithy/protocol/traits/errorfiles/nascent-document-support.errors

@@ -0,0 +1 @@
+[DANGER] smithy.example#DocumentOperationInput$document: Found an incompatible shape when validating the constraints of the `smithy.protocols#rpcv2Cbor` trait attached to `smithy.example#DocumentService`: This protocol does not support document types in most possible scenarios. | rpcv2Cbor.NoDocuments