Adds support for triple quoted strings to the documentation test
generator. Kibana's CONSOLE tool has supported them for a year but we
were unable to use them in Elasticsearch's docs because the process that
converts example snippets into tests couldn't handle this. This change
adds code to convert them into standard JSON so we can pass them to
Elasticsearch.
This commit adds back "id" as the key within a script to specify a
stored script (which with file scripts now gone is no longer ambiguous).
It also adds "source" as a replacement for "code". This is in an attempt
to normalize how scripts are specified across both put stored scripts and script usages, including search template requests. This also deprecates the old inline/stored keys.