Support getting execution by id with specific list of fields

In Mistral, we can get list of executions by endpoint /v2/executions and we can get a single execution by id via endpoint /v2/executions/{id}
In first case, there are lots of ways to configure the output, but none of them is supported in the second case.
I'd like to extend current beavior by adding support to get the specific list of fields.

It is a very common case, e.g. some system wants to check execution state periodically. Instead of loading only state, it downloads a lot of redundant information. Furthermore, this approach spends a lot of traffic, which should be avoided.

So, here's a little Acceptance Criteria:

- Optional 'fields' parameter is supported in Mistral API (/v2/executions) that returns execution by ID
- Mistral returns only list of execution fields that is specified in 'fields' parameter of request
- Mistral returns all fields if 'fields' parameter is absent in request (already exists)