Rework Generics of Transformer interface

[NarekDW]

@snuyanzin could you review generics in Transformer interface, please?

I've found a number of issues here:

1. On the code below, Schema<?, ? extends OUT> means that OUT transform(IN input) function from Field interface could generate the value only of some specific type. For example for CsvTransformer class, OUT is defined as CharSequence, so it means that we can generate CSV only from CharSequence, we couldn't generate CSV (fields) from other objects (integer, double, boolean e.g.).
   Take a look on the test case testCsvWithDifferentObjects and testCsvWithDifferentObjectsFunction.

public interface Transformer<IN extends AbstractProvider<?>, OUT> {
    OUT apply(Object input, Schema<?, ? extends OUT> schema);
...

2. Here, IN generic type should not extends AbstractProvider, because in fact that type corresponds to the
   Schema<IN, ...> interface then it is passed to the function OUT transform(IN input) from Field interface, that function takes an argument of IN type and feeds it to the functional interface, which generates the data based on the argument, therefore, the argument could be an object of any type.
   Take a look on the test case testCsvWithDifferentObjects and testCsvWithDifferentObjectsFunction.

public interface Transformer<IN extends AbstractProvider<?>, OUT> {
    ...
    String generate(List<IN> input, final Schema<IN, ? extends OUT> schema);
    ...
}

3. I think all the functions from interface Transformer should return a data of type OUT

Otherwise, if we don't want to change the types definition, we have to try to adjust the code to that types.

[codecov-commenter]

Codecov Report

Merging #447 (fa62365) into main (35651df) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main     #447      +/-   ##
============================================
+ Coverage     93.00%   93.03%   +0.02%     
- Complexity     2094     2098       +4     
============================================
  Files           233      233              
  Lines          4373     4377       +4     
  Branches        449      449              
============================================
+ Hits           4067     4072       +5     
+ Misses          190      186       -4     
- Partials        116      119       +3     

Impacted Files	Coverage Δ
.../net/datafaker/transformations/CsvTransformer.java	93.10% <100.00%> (+0.51%)	⬆️
...net/datafaker/transformations/JsonTransformer.java	88.04% <100.00%> (ø)
.../net/datafaker/transformations/SqlTransformer.java	66.66% <100.00%> (ø)
.../main/java/net/datafaker/service/FakerContext.java	87.17% <0.00%> (+2.56%)	⬆️

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[snuyanzin]

awesome, thanks