Cleaner enabling of development service

[JoshOrndorff]

Solves #231

This PR continues to improve the development service from both architectural and UX perspectives.

Reduced Code Duplication

On the architectural side, this further reduces duplicated code between the development and full services. There is are no longer separate files for the two services. The two services now share their native_executor_instance! invocation and their entire new_partial functions.

As a side benefit, this also works around an issue I discovered with manual seal where we weren't properly checking inherents paritytech/substrate#8164 That's the issue that caused me to incorrectly conclude that the initial pass at the author filtering was working as expected.

This is still not the end of the road for service code de-duplication. There is more improvement possible, that will come if future PRs.

UX Improvements

This allows users to explicitly request the dev service by using a dedicated --dev-service flag. We are now no longer abusing the --dev flag, so its role can return to being a quick shortcut flag that uses all the right defaults for a simple local one-node network.

We also allow chain specs to request the dev service by using the string "dev-service" in the relay_chain field.

Checklist

• ❌ Does it require a purge of the network?
• ❌ You bumped the runtime version if there are breaking changes in the runtime ?
• ✔️ : Does it require changes in documentation/tutorials ? -- It may. It preserves functionality of the --dev flag, so I don't think anything broke. But there are new nice features that can be documented. I'll discuss all this stuff at our learning session Friday. (cc @albertov19)