rust: add secondary function with preallocated internal vecs

[wlkrm]

Because of its zero-copy properties, flatbuffers might be a good fit for real-time systems.
For real-time systems, memory allocations in critical sections, such as control loops, should be avoided.
Currently in the Rust implementation, I can only preallocate the "data" buffer used for serialization.
This only avoids some allocations at runtime: Some internal vecs of the FlatBufferBuilder are allocated on-demand, which will lead to additional allocations at runtime. It would be useful to be able to preallocate some user-definable space for the internal buffers optionally, as it is already possible for the "data" buffer using the FlatBufferBuilder::with_capacity(usize) function.

Current Behavior

// Schema
namespace users;

table User {
  name:string;
}

root_type User;

let mut buffer = FlatBufferBuilder::with_capacity(1024);
buffer.reset();

loop {
        let name = buffer.create_string("testt");
        let user_args = UserArgs { name: Some(name) };
        let test = User::create(&mut buffer, &user_args);
        buffer.finish(test, None);
        let _ = buffer.finished_data();
 }

This PR

My suggestion is to simply add a second initializer function besides with_capacity(), which also allocated internal capacity, e.g., FlatBufferBuilder::with_internal_capacity(1024 /* data buffer's len */, 16, 32, ... /* internal vecs' lengths */) (Of course with a different len for every vec in the implementation).
Of course, the user would have to make a sophisticated decision on how much to preallocate, but this is already the case for the "data" buffer size.

This PR implements the proposed feature and adds a test, which verfies, that the preallocation is indeed working.
The new API call looks like this:

let mut builder = FlatBufferBuilder::with_internal_capacity(64, 8, 16, 32);

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[wlkrm]

@jtdavis777, here is my PR as discussed in the original issue #8912

[jtdavis777]

Awesome. I'll take a look as I have time and energy 😅

[wlkrm]

Yes that would be great! Thanks

[jtdavis777]

hey @wlkrm, I do not know very much about rust, this does look good and like a good change. I would like to see if there's any good place in the documentation to describe this feature, either in readme's or in the docs that go up on the site.

[wlkrm]

Hey, thanks for reviewing. The docsstrings I added to the new functions will appear in the crates doc. So it will be automatically added here https://docs.rs/flatbuffers/latest/flatbuffers/struct.FlatBufferBuilder.html just like the with_capacity function right now.
What other places for docs do you have in mind?, as this is a feature specifically for the rust implementation, as other languages for example have a garbage collector.

[wlkrm]

hey @wlkrm, I do not know very much about rust, this does look good and like a good change. I would like to see if there's any good place in the documentation to describe this feature, either in readme's or in the docs that go up on the site.

Hey thanks for the review, I have already added docsstrings for the new functions which go up with the rust documentation site of the FlatbufferBuilder struct on the official doc site.
https://docs.rs/flatbuffers/latest/flatbuffers/struct.FlatBufferBuilder.html