feat: config service as param

[zach88]

Resolves #627

What this PR changes:

• Looks for the configUrl parameter in the query string to use it as a custom ConfigService URL.
• Uses the hardcoded ConfigService URL as a fallback if the parameter is not present.

[bh2smith]

This looks good to me. We should wait on @schmanu before merge. Thanks for your contribution here @zach88! 👻🎃🦇

[schmanu]

Hey @zach88,

I am not sure if this could lead to potential attack vectors by providing a malicious config service.
Maybe for now we could extract the config service URL into an .env variable and you could re-deploy the Safe app on IPFS with a different env variable set?

In the meantime I will create an issue to return the transactionService URL in the safe-apps-sdk's getChainInfo function. That would be the securest and easiest way to retrieve the chain config of the Safe's network.

[zach88]

Actually, the url needs to be configured at the config service level, so any safe app that doesn't match the URL will be flagged as not trusted, just like it does for custom safe-apps. however, exposing the config service through the safe-app-sdk would be a nice workaround. thanks @schmanu