[dev-approuter] Allow to start CAP server with cds-ts serve or any other command

[tiosteel]

Is your feature request related to a problem? Please describe.
I want to use the dev-approuter in combination with monorepo and TS on both SAPUI5 and CAP sides for local development.

I'm almost there:
https://github.com/tiosteel/mortgagecalc
(see srv/cap-server and app/router)

The only thing which prevents me from finalising it is a fact that dev-approuter cannot start my server with cds-ts serve.

Describe the solution you'd like
a. allow to provide a console command to start CAP server
b. allow to specify if I need the exact cds-ts to start it

Describe alternatives you've considered
a. Use regular approuter + cap server with 2 different active consoles.
b. precompile ts on every restart (which cds-ts usually allows to avoid)

Additional context
inspired by this:
@nicoschoenteich
https://www.youtube.com/watch?v=bJIMIE7yjIE
https://github.com/SAP-samples/sap-tech-bytes/tree/cloud-foundry-basics/post6

[petermuessig]

@nicoschoenteich - maybe something to look into from your side?