Pass they keys in the initialisation of the SDK

[dayerdl]

Describe the feature

At the moment the keys for Branch are exposed in the gradle/plist files or in the config.json file

Describe the solution you'd like

Allow something this

Branch.getInstance({BRANCH_API_KEY})

So the key is not exposed on the branch.json file

Can you submit a pull request?

Yes

[RodrigoSMarques]

This involves a delay in the SDK initialization.

This delay has had an impact in the past.

It was discussed here: #450

[RodrigoSMarques]

I tried implementing this solution via .ENV or dart-define, but I couldn't find a simple solution that works for both iOS and Android.

A plugin does not have access to the project's environment variables.

The fact is, when the application starts, the native SDK will be initialized before Dart/Flutter, and the key value should already be configured.