[v6] Routers could accept history as optional prop

[Bekaxp]

Issue

We have a widget system where the history needs to be shared between all of them. Currently the new Routers (Browser, Hash and Memory) do not have any option to pass an already existent history, which will be used instead of always creating a new one.

Suggestion

Is there a good reason why not extend these Routers with an additional history prop, which will be used in the following lines:

• For MemoryRouter: https://github.com/ReactTraining/react-router/blob/262b45dd8d6cbdc3a984259c5591ffc3de80d600/packages/react-router/index.tsx#L124
• For HashRouter: https://github.com/ReactTraining/react-router/blob/262b45dd8d6cbdc3a984259c5591ffc3de80d600/packages/react-router-dom/index.tsx#L139
• For BrowserRouter: https://github.com/ReactTraining/react-router/blob/262b45dd8d6cbdc3a984259c5591ffc3de80d600/packages/react-router-dom/index.tsx#L95
  like this:
  history || createXXXHistory()

Thank you.

[timdorr]

Duplicate of #7585 and has a PR open to resolve this: #7586