docs: add OneofValue type helper

[clucle]

ref: #1023

In previous helper, it is not possible to construct a message with the oneof field passed as a function argument.

This helper allows for strict type checking and construct message using oneof fields as an argument.

Example

syntax = "proto3";

package logs;

message SignUp {
    string name = 1;
}

message Login {
    string account_id = 1;
}

message Logout {
    string account_id = 1;
    string reason = 2;
}

message LogPayload {
  string created_at = 1;
  oneof payload {
    SignUp signup = 2;
    Login login = 3;
    Logout lougout = 4;
  }
}

// pass by argument
foo<P extends PaloadNames>(
  payload: OneOfValue<LogPayload['payload'], P>
) {
  const req: LogPayload = {
    createdAt: new Date().toUTCString(),
    payload,
  };

  send(req);
}

// Usage

// Good
this.foo( {
  $case: 'login',
  login: {
    accountId: 'accountId',
  },
});

// Build Fail because case and field is not matched
// Argument of type '{ $case: string; login: { accountId: string; }; }' is not assignable to parameter of type 'never'.
this.foo( {
  $case: 'logout',
  login: {
    accountId: 'accountId',
  },
});

[stephenh]

Nice, thanks!

[stephenh]

🎉 This PR is included in version 1.173.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release
• v1.173.0

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