Add operationId on OpenAPI operations

[Lucidiot]

This pull request adds an operationId property on generated OpenAPI operations from #6532. This property is recommended in the OpenAPI specification and is actually a requirement if we want the generated schema to have any use with OpenAPI clients ; most client libraries will use those operation IDs to build methods. See the Operation Object in the OpenAPI spec for more info.

Example with openapi3 and an hypothetical ThingsListAPIView:

>>> import yaml
>>> from openapi3 import OpenAPI
>>> api = OpenAPI(yaml.safe_load(open('schema.yml').read()))
>>> api.call_ListThings()
[{'id': 1, 'name': 'thing1'}, {'id': 2, 'name': 'thing2'}]

Same example with apistar:

>>> import apistar
>>> api = apistar.Client(open('schema.yml').read())
>>> c.request('ListThings')
[{'id': 1, 'name': 'thing1'}, {'id': 2, 'name': 'thing2'}]

TODO:

• Probably more unit tests
• Allow setting a custom operation ID on views with an attribute?
• Raise exceptions when operation IDs are not unique across a schema

Fixes #6542

[carltongibson]

Hi @Lucidiot. Super.

Let's merge this in as-is, and address the points on the main PR. (It gives people a chance to test...)
I'll have a look over the weekend in more depth, but I'd rather have it in attracting issues than think about it forever first. (I hope that makes sense.)

Thanks for the input!