Let user upload chainSpec and wasm blob

[amaury1093]

By default, we'll bundle the Kusama wasm blob inside the extension.

• Step 1. Users should be able to upload their own chainSpec via the UI.

start_client takes a chain_spec arg, we should make that arg take a JSON chainSpec. We'll just run in this case the kusama wasm blob with the inputted chainSpec.

• Step 2. Users should be able to upload their own wasm blob via the UI.

Sometimes the chainSpec is not enough (e.g. when the custom chain implements custom BlockNumber type, Header etc, which happens at compile time), so the user needs to have a possibility to upload a wasm blob too, and let the extension run that custom wasm blob.

reported by @expenses

[expenses]

Step 1 has just been done ;^)