feat: enable ext communication helper

[weichweich]

related KILTprotocol/ticket/issues/2126

This PR adds a helper to signal the extension that the DApp supports the credential API.

The Credential API version is hard coded inside this library as it is expected that downstream projects support exactly the version that is supported by this library. If this library updates the Credential API version all downstream project will be required to update their code to support the new version.

Support for possible other APIs

If there are new APIs added to the wallets this method can be extended in the following way:

export function initializeKiltExtensionAPI(options: {
  credentialApi: boolean,
  newApi: boolean,
} = {
  credentialApi: true,
  newApi: false,
})

This would be a non breaking change.

[arty-name]

Should the specific version of the API be customizable? We also have 3.1 now and 3.2 is in the works.

[weichweich]

I think we would always update this library with the new types/changed API. So if you want to support 3.1 or 3.2 you would update this library and get the changes as part of the library.

[arty-name]

That could be rather inconvenient for the application developer who wants flexibility