Make SDK extensible

[Dr-Electron]

Description

For external libraries/applications to use the SDK for their specific use case it would be nice to have interfaces in the SDK that can be extended.

Examples:

• Extend accounts to have custom send functions
• Extend the Secret manager
• Extend storage (custom DB)

Motivation

• Use cases like identity often have their own key management like we can also see here.

• Some use cases might need different storage interfaces like we can see here.

Open questions (optional)

Tbh I have no clue how complicated this would be for bindings...

Are you planning to do it yourself in a pull request?

No

[Thoralf-M]

Please open own issues for specific things to do

[Dr-Electron]

I just wanted to get your opinion first as see (now) that there are a lot of moving components. Seems like you already (are working on) use traits in different places so that should be nice.

What about bindings? Should we find a way to extend those?

[Thoralf-M]

For things where we have traits we could maybe implement them from bindings via closures. I did some experimentation with the StorageAdapter trait and nodejs here https://github.com/thoralf-m/iota-sdk/commit/bf43fb3ea94016e8d71830bd8aae386ca2ec8624