Support field mask for repeated struct

[sysulq]

Summary

We need support select specified field for the slice of struct, which would be great.

Why do we need this?

message demo {
  repeated A a=1;
}

message A {
  int32 a=1;
  int32 b=2;
}

In some situations, we need to support return the specified b field to clients only.

What is already there? What do you see now?

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What is missing? What do you want to see?

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Environment

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How do you propose to implement this?

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Can you do this yourself and submit a Pull Request?

[rvolosatovs]

Sounds good, are you willing to contribute this feature?

[sysulq]

I wanna to try this, but I have no time recently...