Add user-defined function support in SQL Client

This issue is a subtask of part two "Full Embedded SQL Client" of the implementation plan mentioned in FLIP-24.

It should be possible to declare user-defined functions in the SQL client. For now, we limit the registration to classes that implement ScalarFunction, TableFunction, AggregateFunction. Functions that are implemented in SQL are not part of this issue.

I would suggest to introduce a functions top-level property. The declaration could look similar to:

 
functions: 
  - name: testFunction 
    from: class <-- optional, default: class 
    class: org.my.MyScalarFunction 
    constructor: <-- optional, needed for certain types of functions 
      - 42.0 
      - class: org.my.Class <-- possibility to create objects via properties 
        constructor: 
          - 1 
          - true 
          - false 
          - "whatever" 
          - type: INT 
            value: 1