Return results in JSON format with serde

[ch1ru]

The purpose of this PR is twofold:

• To return results in JSON format instead of printing in mock-json style to console
• Return with a result type for all functions instead of just printing the error to console

Currently a lot of the responses can only be viewed in the terminal and not all functions fail on an error, instead just printing out the error and returning. It would also be difficult to use the sample node non-interactively, for example via http responses. Serde_json offers consistent formatting for responses and is already a dependency in the sample node.

The PR has the following advantages:

• Responses are in a format that can be parsed more easily. For example if implementing an API, definitive responses on channel info, connected peers etc. can be sent remotely
• Better error handling since most errors are now returned as a result rather than just been printed to console
• Improved readability, printing JSON brackets no longer necessary
• Easier to embed objects in responses, for example sent and received payments are now separate return fields

[tnull]

Mh, I'm not sure we want to go this way. ldk-sample is really meant as an educational example to showcase how the different modules of LDK fit together, i.e., it's primary goal is to be read, even more so than to be run. So the more boilerplate and complexity we add, the more the reader has to parse and understand.

Moreover, I really don't think we want to optimize our logging outputs w.r.t. machine readability. If you need to interact with the code non-interactively, you should specify and build-out some kind of API, not just parse in log messages.

[ch1ru]

Fair enough I can understand that. Personally though I find this slightly more readable even if it adds some extra lines of code. I just thought that if it's returning in json format it would be useful to return an actual json object.

[tnull]

It's actually not returning JSON, but simply the default Debug output of the Rust objects.

[ch1ru]

I didn't know that, guess I'll have to look more into that. Thanks