feat: add types for cds.build

[stockbal]

Adds types for cds.build to have proper type support when writing a custom CDS build plugin

[daogrady]

Hi Ludwig,

thanks for this contribution! We're actually currently looking into how we can properly offer cds-dk types, hopefully without tainting the core cds types and causing too much confusion. I see that you have pointed out that the types are only available from dk in the comments, but I'd still like to put this on hold until we have figured some things out.

Best,
Daniel

[stockbal]

Hi Daniel,

sure thing. It's not that high on my requirements list. I only created this PR because of the typescript issues in cds-plugin.js of cds-typer. I wanted to create another for cds.add as well, but put it hold because for cds.add the docs explicitly mention the use of @sap/cds-dk.

When you figured out how and where to add the dk-specific typings, just give a quick shout out, I am happy to do it 😊

Regards,
Ludwig