Allow extension of RestApiContext

[richardgaunt]

I would like to be able to extend the RestApiContext class to make more specialised contexts for my use case.

My particular use case is communicating with OpenSearch, the requests for this API require me pausing for a second before making the request and also a step that clears cache etc that I would like to bundle up into a context but rely on RestApiContext underneath it.

I would like to be able to create my own custom steps for making requests but also have the assertions that RestApiContext provides (as they are great). At the moment I can make the request but I am unable to access the private properties and private methods within RestApiContext