[Java] add jackson annotations to setters

[SoMuchForSubtlety]

Closes #6856

Jackson deserialization fails with the message "Problem deserializing 'setterless' property" if json keys are not in camel case. This commit fixes that problem by adding @JsonProprety to setters.

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@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @nmuesch

[SoMuchForSubtlety]

Looks like this creates an issue when JsonNullable is also used:

  @JsonProperty(JSON_PROPERTY_NULLABLE_SHAPE)
  public void setNullableShape_JsonNullable(JsonNullable<NullableShape> nullableShape) {
    this.nullableShape = nullableShape;
  }

  @JsonProperty(JSON_PROPERTY_NULLABLE_SHAPE)
  @JsonInclude(value = JsonInclude.Include.USE_DEFAULTS)
  public void setNullableShape(NullableShape nullableShape) {
    this.nullableShape = JsonNullable.<NullableShape>of(nullableShape);
  }

com.fasterxml.jackson.databind.JsonMappingException: 
Conflicting setter definitions for property "nullableShape": org.openapitools.client.model.Drawing#setNullableShape(1 params) vs org.openapitools.client.model.Drawing#setNullableShape_JsonNullable(1 params)

two possible solutions are:

• add jackson annotations to fields instead of methods
• only add jackson annotations to the regular setter if there is no JsonNullable setter

[SoMuchForSubtlety]

I implemented option two.

[SoMuchForSubtlety]

@wing328 any chance of getting this into the next minor release?

[wing328]

LGTM but I've not tested it locally