Added support for generation of mutable structs

[Ochibobo]

An issue was opened here by @dourouc05 that requests support for a mutable API. This PR attempts to address that.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.93%. Comparing base (2e80936) to head (2c8b303).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #259      +/-   ##
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- Coverage   92.99%   90.93%   -2.07%     
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  Files          25       25              
  Lines        2812     2636     -176     
==========================================
- Hits         2615     2397     -218     
- Misses        197      239      +42     

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[mrkn]

@Ochibobo I love this feature.

In addition, what do you think about adding a new boolean option called julia_mutalbe_struct to specify this flag in the option directive, as shown below?

syntax = "proto3";

option julia_mutable_struct = true;

message Message {
    string message = 1;
};

[Drvi]

Thanks! The overall idea looks good to me!

Let's shorten the name to mutable_structs and let's mention that this changes the default struct "kind" -- in the future we might want to add more granularity for controlling this via comments etc.

I also think we should add the word mutable to the JULIA_RESERVED_KEYWORDS list in src/codegen/names.jl

Suggested change

	- `with_mutable_structs::Bool=false`: If set to `true`, generate structs that are mutable in nature.
	- `mutable_structs::Bool=false`: If set to `true`, the generator will default to generating mutable structs.