feat: peek/modify decoded unknown fields

[ii64]

Is your feature request related to a problem? Please describe.

We've got CarryingUnknownFields to see if there's unknown fields, but what we could do with it besides reflecting them back to wire data? Currently, _unknownFields is not exported, and no methods expose it. So technically, there's no way to touch them without going to unsafe.

Describe the solution you'd like, and alternatives you've considered

Expose _unknownFields field, or provide a method to expose it.
In addition, since field ids can be unordered, implement new method on type unknown.Fields to find the field id and field type, or matcher that calls back whether there's matched field id and field type.

Additional context

No additional context.

[lsjbd]

But what you want to do with those unknown fields? They are basically not supposed to be exposed to users.

[ii64]

Thank you for replying. I was thinking about google.protobuf.Any implementation for Thrift. These unknown structure/data will be encoded to unknown fields along with the type metadata.
EDIT: So in order to do that we need to access unexported _unknownFields.