Add support for repeated message fields.

[mennanov]

Add support for repeated message fields so that if given a Profile message:

message Photo {
  int64 photo_id = 1;
  string path = 2;
  Dimensions dimensions = 3;
}

message Dimensions {
  int32 width = 1;
  int32 height = 2;
}

message Profile {
  User user = 1;
  Photo photo = 2;
  repeated int64 login_timestamps = 3;
  repeated Photo gallery = 4;
}

and a mask like "gallery.photo_id", "gallery.dimensions.height" the Filter() function will keep those fields in the repeated field gallery.

[codecov]

Codecov Report

Merging #2 (a860b57) into main (9e8bf86) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main        #2   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           51        62   +11     
=========================================
+ Hits            51        62   +11     

Impacted Files	Coverage Δ
fmutils.go	100.00% <100.00%> (ø)

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[SachsA]

Thank you for this update! It works like a charm!