Auto-install Rust compiler toolchain locally for CI and JS devs of other projects

[peaBerberian]

Since we brought the WebAssembly MPD parser to fix performance issues we had with some contents at canal+, we added another language (Rust) to the project which is not compatible with most of the JS-world tools we and other JS developers usually deal with.

Now that we added the MULTI_THREAD experimental feature and embeds, the WebAssembly generation from that other language is even a required step in our main build script (npm run build).

Consequently, when another developer or a CI needs to build an RxPlayer (e.g. to test a development before a release is made), they have to install:

1. NodeJS + npm
2. our JavaScript dependencies
3. A Rust toolchain able to output WebAssembly. We generally go through rustup here basically an helper tool to install Rust toolchains
4. binaryen, which is a collection of tools to transform that WebAssembly file.

(1) and (2) are very common use cases that both JS people and CI workflows do very easily: they mostly already have nodejs + npm installed locally (or it's very simple to set-up) and the second is only an npm install call away.

(3) and (4) however, though not hard (you have to install rustup and binaryen which is pretty simple on all platforms we relied on), are more unusual dependencies that JS developers might be reticent to install and that are more work to set-up on a CI (it's fine on ours, but we also sometimes want other projects, with their own CI, to test our developments for their use cases).

Consequently I'm doing here a proof-of-concept where those tools are auto-installed locally if not found when doing an npm run build call, letting both people and CI produce a build easily without having to deal with the dual-language complexity.

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For rustup it's pretty straightforward as their install script already take care of the platform detection, WebAssembly target installation and so on. All I had to do is to make sure binaries were installed locally (./tmp) and did not pollute the developers' environment (e.g. no PATH modification).

For binaryen however, it's less simple as they don't propose such auto-install system.
However, I noticed that they propose prebuilt binaries for the most usual platforms on their github's release page:
https://github.com/WebAssembly/binaryen/releases

The idea here (not yet finished), is to automatically detect the platform of the user and fetch the right binaryen binary locally to then rely on both it and the local rustup to produce builds in case the command isn't available globally.

Also, because it will be faster and less error-prone for frequent developers to install both of those globally once and for all, we print a notice explaining what we do when that auto-install script is run.

[Florent-Bouisset]

👍