Introduce Try*() Accessor Method Generation for One-Of Groups

[danilvpetrov]

What change does this introduce?

This PR introduces accessor method generation for each type in "one-off" groups for protobuf messages. While protoc-gen-go plugin already generates accessor methods for each type in "one-of groups", those generated methods lack the ability to differentiate between the absence of a value and the presence of a zero value.

The new generated methods have the following use pattern, where the returned ok boolean value signifies if the target value is set in "one-of" group. If ok is false, the caller can disregard the value of foo variable as it would not be originally set in the one-of group.

    foo, ok := message.OneOf.TryFoo()

Why make this change?

This change hopefully make developers' life simpler by introducing accessor methods on "one-of" groups that can be used deterministically to retrieve the desired value.

Is there anything you are unsure about?

No

What issues does this relate to?

N/A