DID SDK compatibility improvements

[AlexanderShenshin]

Thank you for your great work!
There is a list of problems that impact DID SDK functionality in non-blocking way - it will be great to resolve these to improve compatibility.

The most significant point here is support for ECDSA key type:

• Current to_bytes_raw method does not work for ECDSA
  • Raw format seems to be not supported for ECDSA key dumping (see hazmat.cryptography docs)
• ECDSA raw public key parsing is not implemented (mentioned in recent PR)

There are other issues that are not critical, but nice to have (and will make library more robust):

• No dispose/close for client class - we probably want to close gRPC channel
• Related to TopicMessageQuery
  • Topic subscription is not cancellable
    • Java SDK provides a subscription handle that can be used to cancel operation (for example, on error)
    • Java SDK example
  • There is no completion handler for topic subscription, so the caller do not know when all messages have been received
    • "all messages received" is a relative statement, of course, but it will be good for SDK to provide some default logic on that
    • Java SDK example
  • Some query properties/settings are missing comparing to Java SDK
    • MaxBackoff
    • MaxAttempts

[nadineloepfe]

will try to pick this up Thursday or Friday

[nadineloepfe]

PR here:
#50

@AlexanderShenshin

[AlexanderShenshin]

@nadineloepfe PR looks good, thank you!

We can close this after PR is merged, I will do more testing of these changes in DID SDK and get back to you if I encounter any problems.