Split migration api endpoint into service specific one

[TheFox0x7]

Feature Description

I think api could benefit from having a more narrow endpoints for migration. Instead of a single which has a lot of options - some irrelevant to the service chosen a service specific one would allow for better validation of the request as it can take service into account instead being generic.

My proposal is to add dedicated routes to /api/v1/repos/migrate/{service} endpoint (where service is a known migration source) and deprecate the generic endpoint.

This would also help users in understanding the endpoint - which options are supported and which aren't, instead of having to guess or go to UI to figure it out.

Screenshots

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