Stream transaction support in the Collection and Graph APIs

[tjoubert]

Solves issue #328

• Added ApiHeaderProperties allowing users to pass any header (as long as it is supported) to a method. This will be useful for situations in the future where the API adds new headers and we have not had the time to add named properties/parameters for these new headers in the driver.
• Added stream transaction support in the Collection and Graph APIs

[rossmills99]

Nice idea to add support for additional headers. I realise it looks quite trivial but wonder if we should consider having one or mote simple unit tests for ToWebHeaderCollection method?

[rossmills99]

Looks good - I made a comment that you could consider adding a unit test for the new ToWebHeaderCollection method but I don't think it's a blocker.

[DiscoPYF]

Very nice! Approved 👍
I agree with Ross' comment that it would be good to add tests to cover any new methods we add, but not a blocker.

With the new ApiHeaderProperties class you created, we could open a follow-up enhancement to add it to every API methods to support general headers, such as x-arango-async.

Another tought: I wonder if we should create dedicated classes e.g. GraphHeaderProperties and CollectionHeaderProperties to prevent breaking changes in future if headers specific to those APIs are introduced and we then need to define those classes. Not a blocker.

Let me know what you think. Happy to see this merged in any case. 🚀