Optimise coins asset files

[takenagain]

Optimise repository size by removing coins asset files and replacing them with a Python script that downloads the assets in CI/CD pipelines and local developer build steps.

Changes:

• Add a Python script to download the coins asset files from the coins repository.
• Remove the copies of the coin asset files from this repository to reduce the repository size.
• Update the GitHub build-android action and the README.md to use the Python script to download the coins asset files.

[CharlVS]

@takenagain thanks, it's looking good. Can you write a Flutter test that checks that all loaded coins have a corresponding coin icon? You won't be able to simply scan through the json coin data files and check that each entry has a matching file in the icons folder. You will need to use the app's logic to filter out coins that are not shown in the app (not all coins are supported) and then use the app's function for mapping coin ticker to the icon file name.

[CharlVS]

@takenagain we have the runtime update parameters specified here.

Please make the following changes:

1. Refactor the coins_updater.dart and your CI script to read from a revised coins_ci.json.

The revised coins_ci.json file should have the following format. Feel free to improve on it if you see any opportunities.

{
    "bundled_coins_repo_commit": "8cb5983ab858dde915e8de3175c15cd0f5572356",
    "coins_repo_url": "https://raw.githubusercontent.com/KomodoPlatform/coins",
    "coins_repo_branch": "master",
    "runtime_updates_enabled": true,
    "mapped_files": {
        "assets/coins_config.json": "utils/coins_config.json",
        "assets/coins.json": "coins"
    },
    "mapped_folders": {
        "assets/coin-icons/": "icons/"
    }
}

2. Update the bundled commit hash in coins_ci.json when the script runs. If the script is running in a GH action then fail the check if the downloaded files' commit doesn't match the commit hash in bundled_coins_repo_commit. I.e. There are uncommitted changes to coins_ci.json after the script runs.

[takenagain]

This approach was abandoned in favour of asset transformation, which is integrated into Flutter's build process.