Add more hooks to FlowRunner.initialize_run()

[jlowin]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• updates CHANGELOG.md appropriately
• is ready for review

Depends on #546

What does this PR change?

• In preparation for more efficient ways of handling states provided by Prefect Cloud, this PR adds two new arguments to FlowRunner.initialize_run(): task_states and task_contexts. This gives the initialize_run method an opportunity to modify task-specific inputs. In this PR, it is only used to put some items in context (such as the task name and id)
• Adds new "return types" for FlowRunner.initialize_run and TaskRunner.initialize_run -- this is simply a convenience to make it easier to statically type the library and access returned values
• Adds tests for initialize_run() (it was lightly tested before)

Why is this PR important?

This PR stands alone by providing improved opportunity to interact with task states and contexts before a flow runs, but will mainly be used in a forthcoming PR that loads all task states from Cloud in a single call before the Flow runs. These hooks will make it possible to provide those states to the tasks.

[cicdw]

Some doc changes requested

[cicdw]

LGTM I approve