feat: add preflight method to ManagerAPI

[secustor]

Changes

Adds a new async preflight function to the ManagerAPI.

Kept it as single method for simplicity and I can not come up with an use case where this would be helpful.

Context

This allows to modify Renovates configuration based on content of the repository or other existing configuration.

Closes #29809

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How I've tested my work (please select one)

I have verified these changes via:

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[viceice]

we need to think about config caching behavior. do the hashes match? I think they are checked before extraction.

this new function needs a git sync too.