Refactor: Update the parameters for DWDS.start

[elliette]

This PR:

• refactors the parameters for Dwds.start so that all of the miscellaneous strings / boolean parameters are collected into either DebugSettings or AppMetadata. This is to make it easier to add optional parameters to the Dwds.start method, since we won't need to update the flutter_tools DwdsLauncher each time: https://github.com/flutter/flutter/blob/367203b3011fc1752cfa1f51adf9751d090c94e6/packages/flutter_tools/lib/src/isolated/devfs_web.dart#L46
• sets DebugSettings and AppMetadata as globals so that they can be accessed from anywhere (instead of having to pipe some of their fields down to the Injector as we were previously doing)
• renames the parameter names to consistently use devTools/DevTools instead of devtools/Devtools
• makes the parameter enableDebugging is no longer required but optional. If not set, it defaults to true

This is preparatory work for #2198, because we will be adding a new parameter to Dwds.start (the workspace name)

[annagrin]

Thanks Elliott, really appreciate cleaning dwds.start up! I think we have and issue for this work somewhere, will try to find it an update. Left a few comments.

[annagrin]

Thanks, left some more comments

[annagrin]

Can all those fields be final, or late final if needed? I don't think we want to allow changing them during the run of dwds?

[annagrin]

LGTM but would lie a follow up PR where we have pre-defined test configurations and/or helpers to create them. Consider passing tool configuration to contest constructor as well.