feat: adds global config experimental flag

[douglance]

In order to be able to ship experimental features for broader testing among consuming applications, we've added a global config object with an experimental flag that would be used like this:

import {
  enableExperimentalFeatures,
  experimentalFeaturesEnabled
} from '@arbitrum/sdk'


// To enable experimental features:
enableExperimentalFeatures()


// To check if experimental features are enabled anywhere in the application
if (experimentalFeaturesEnabled()){
  // do experimental things
}

I did it this way to limit access to the global config object as much as possible.