Add ExecutionContextSerializer implementation based on Gson

[fmbenhassine]

The goal of this issue is to add an implementation of ExecutionContextSerializer based on Google's Gson, to give our users the choice between at least two implementations (ie Jackson and Gson) out-of-the-box, as mentioned in #4140.

[fmbenhassine]

For some types, a serialization round trip with gson does not work OOTB. The following tests are failing with gson 2.9.1:

@Test
void testGsonSerializationRoundTripForDate() {
    Gson gson = new Gson();
    Date src = new Date();
    String s = gson.toJson(src);
    Date date = gson.fromJson(new StringReader(s), Date.class);
    Assertions.assertEquals(date, src);
}

@Test
void testGsonSerializationRoundTripForLocalDate() {
    Gson gson = new Gson();
    LocalDate src = LocalDate.now();
    String s = gson.toJson(src);
    LocalDate date = gson.fromJson(new StringReader(s), LocalDate.class);
    Assertions.assertEquals(date, src);
}