Add two new methods to ReceiveMessageBuilder in the Java DSL: validateXpath and validateJsonPath, which accept Maps as arguments.

I propose to add two new methods to ReceiveMessageBuilder in the Java DSL: validateXpath and validateJsonPath, which accept Maps as arguments. (see lines 585-604)

Here is the motivation... I have many test cases similar to this:

```java
designer.soap()
.client(this.client)
.receive()
.schemaValidation(false)
.validate("//lexs:ResultCode", "Success")
.validate("//lexs:SearchResponseMetadata/lexs:MaxItemMatchesRequested", "10")
.validate("//lexs:SearchResponseMetadata/lexs:NumberItemMatches", "10")
.validate("//lexs:SearchResponseMetadata/lexs:ServerLimitIndicator", "true");
```
where the only difference is a variable number of assertions. What motivates the need for the new methods is the desire to create a single method in my test code that eliminates redundancy in individual tests, like this:

```java
protected void runTest(TestDesigner designer, String inputPathString, Map assertions) {
...
designer.soap()
... (as above)
.validate(assertions); <--new method used here
}
```

I found no test cases for this class and therefore created a JUnit 5 test class that covers the public methods as well as the two proposed new methods. There are four tests I left in Disabled state - I was unable to get the desired outcome using Mockito.

There are two other code changes that are unrelated, and that could be rejected or deferred.
`getScriptValidationContext` throws a `NullPointerException` if `messageType` has not previously been called. For this I added an assertion similar to those on some other methods.

A couple of methods related to `headerFragment` proved difficult to mock and test because a `StringResult` is created inside the method. I introduced helper methods that could be called by the unit tests, but left the existing public method signatures unchanged. These methods delegate to the helpers. Again, these changes were introduced only to facilitate unit testing.

As directed, I've created this issue in relation to this pull request: citrusframework/citrus#567