feat: load zkey from a file path, add file downloader in swift

[vivianjeng]

Research for #65

TODO

• [ ] use #[cfg(feature = "use_external_zkey")] to control features
• [x] download and load arkzkey in initializeMopro (See 3.)
• [ ] download file in android and use it in proof
• [ ] benchmark different size of arkzkeys
• [ ] try on real device

1. load from a fixed path e.g. "mopro-core/examples/circom/keccak256/target/keccak256_256_test_final.arkzkey" proving time in swift

   
   Time to deserialize proving key: 9.177727583s
   Time to deserialize matrices: 70.397416ms
   Initializing arkzkey took: 9.25s

Generating proof 2 Witness generation took: 423.89ms Loading arkzkey took: 26.66ms

proof generation took: 1.36s

> there is no much difference between using `ARKZKEY` and `.arkzkey`
> Using `ARKZKEY` is even slightly slower than loading from a `.arkzkey` file

2. 
download from a server `e.g. "https://mopro.vivianjeng.xyz/keccak256_256_test_final.arkzkey"`
```js
Download ark key took: 0.021011948585510254

Time to deserialize proving key: 8.894824041s
Time to deserialize matrices: 26.038125ms
Initializing arkzkey took: 8.92s

Generating proof 2
Witness generation took: 410.84ms

// TODO: should be done in initialization
Time to deserialize proving key: 8.895085333s
Time to deserialize matrices: 24.071833ms
Loading arkzkey took: 8.94s
proof generation took: 979.47ms

3. Init with arkzkey initialization

Time to deserialize proving key: 8.930367541s
Time to deserialize matrices: 24.104291ms

Generating proof 2
Witness generation took: 429.28ms
Loading arkzkey took: 208.00ns
proof generation took: 963.43ms

Verification time 2: 3.227791ms

[oskarth]

What's needed for this PR to be ready for review/merge? Can we gate this feature behind a feature flag maybe? So existing flow works but this can be toggled. Also basic docs in README or so would be great

[oskarth]

@vivianjeng can this PR be closed with #75 merged?