Add support for specifying language

[davsclaus]

for the steps such as filter, choice etc its hardcoded to jsonpath as the languge.

We should allow to define what language we want to use, so we can do simple, xpath and others as well

[davsclaus]

You can now specify the language

flows:
- steps:
  - kind: "endpoint"
    uri: "direct:start"
  - kind: "filter"
    expression: "${body} contains 'James'"
    language: "simple"
    steps:
    - kind: "endpoint"
      uri: "mock:matched"
  - kind: "endpoint"
    uri: "mock:allMessages"

The default language is still jsonpath