Add ability to save context to streams

[pjvds]

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This adds the ability to save an api context to a stream, rather than a file. This makes the save more flexible and removes the dependency to the file system for saving and loading contexts.

The pull requests comes with extra unit tests ✅

[OGKevin]

@pjvds thanks for this PR, did you read https://raw.githubusercontent.com/bunq/sdk_csharp/develop/.github/PULL_REQUEST_TEMPLATE.md ?

Could you please explain this change in an issue and rebase your commits to link to this issue. Your commit message should pass the following regex: https://github.com/bunq/sdk_csharp/blob/f48a2422dad82ee87bff27685eb72cf44b09516c/.zappr.yaml#L4