Create: Allow to pass in transient data on instance creation

[iLLiCiTiT]

Changelog Description

It is possible to set transient data on instance creation.

Additional info

Create plugins cannot fill attribute definitions on instance initialization, if their logic is based on transient data.

Testing notes:

1. Validate code changes.

[BigRoy]

Flawless - seems to work perfectly.

I do think for the transient_data I'm not entirely sure whether the key is required to be a str because it'd support any key a regular dict would support? Although I don't think the type hint is problematic in practice.

[iLLiCiTiT]

I'm not entirely sure whether the key is required to be a str because it'd support any key a regular dict would support?

I would say it is reasonable to not do that at the first level of transient data.