Allow setting dart-defines for the server executable.

### Description

Currently there is no way to set `--dart-define`s when firebase compiles the dart function to an executable.

### Use Case

Generally setting const values for Dart.

Also using Jaspr with firebase functions, which needs some framework specific defines.

### Proposed Solution

It is unclear to me how functions are built so idk, I couldn't find any documentation regarding this.

### Alternatives Considered

### Additional Context