any: read a type URL without a '/' as a bare message name

[iainmcgin]

A google.protobuf.Any whose type_url has no / was resolved by DynamicMessage and rejected by the generated Any JSON parser. any.proto requires the /; protobuf-go and Python read a bare full name anyway, and C++ and Java reject it.

• The generated Any JSON parser accepts {"@type": "pkg.Message", ...} when the type registry has a JSON entry for pkg.Message. Such an Any is now written expanded; it was written as base64 under value, which the same parser rejected. A payload that does not decode as that message now fails to serialize.
• A bare name that does not resolve is a parse error and never takes the base64 form. The conformance test AnyWktRepresentationWithBadType requires that. The serializer still writes the base64 form for such an Any, so it does not round-trip.
• AnyRegistry::lookup and TypeRegistry::json_any_by_url find an entry by a bare name, and an entry registered under a bare name is found under any URL prefix.
• Any::type_name, is_message and unpack_message (unreleased, any: add typed pack and unpack helpers #499) read a URL without a / as the name.
• JsonParseOptions::strict_any_type_urls and DynamicMessageSeed::strict_any_type_urls reject an @type without a /, at any depth of Any in Any. The reflective seeds pass both parse options down in one ParseFlags value, which is most of the change in reflect/json.rs.

let opts = JsonParseOptions::new().strict_any_type_urls(true);
let msg = with_json_parse_options(&opts, || serde_json::from_str::<MyMsg>(json))?;

Serialization, DynamicMessage::unpack_any and Any::type_name take no options and resolve a bare name. Text format does not resolve a bare name, because [pkg.Message] { ... } is the syntax of an extension field.

The error for an empty @type, or one ending in /, now reads @type "..." is not a valid type URL: it names no type.

The changelog entry is Changed. #485, which changed the same output for a URL with an unregistered prefix, is filed under Breaking changes; 0.9.2 could not parse a bare @type at all, so less depends on the old output here.

Supersedes #524, which made the reflective path reject a bare name.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}