APIAggregator#PrepareRun should return exported type

[tedyu]

What would you like to be added:
Looking at the usage of APIAggregator#PrepareRun, the first return value is used as runnable.
We should export runnable type (Runnable) and declare the return type for APIAggregator#PrepareRun as Runnable.

Why is this needed:
APIAggregator#PrepareRun returns preparedAPIAggregator which is unexported.

[tedyu]

/sig api-machinery

[liggitt]

cc @sttts @deads2k

[yue9944882]

this is a deisgned "option ----> config --(complete) --> completedConfig --(new)--> instance" code flow. the other way round, i remember we agreed refactoring and applying it throughout the codebase somewhere else offline..

[fedebongio]

/assign @sttts

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