Optional parameters in DaphneDSL UDFs

User-defined functions in DaphneDSL should support optional parameters with default values.

Example:
```
def foo(x, y = 0) {...}
def bar(x, y:si64 = 0) {...}
def baz(x, y:matrix = seq(1, 10, 1)) {...}
```

The issue names no files, tests, or entry points. Start by locating the DaphneDSL UDF implementation and use the three examples as acceptance cases: untyped, typed, and expression-valued default parameters should all be supported.

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