Support serving multiple functions at once

[soedirgo]

Feature request

Is your feature request related to a problem? Please describe.

Right now it's not possible to serve multiple functions at once.

Describe the solution you'd like

Support supabase functions serve (with no function name arg) which serves all functions in supabase/functions.

Describe alternatives you've considered

None

Additional context

Asked in https://github.com/supabase/cli/issues/456#issuecomment-1248002166