SendTransaction: add support for specific schemas

[shjortConcordium]

Purpose

Update schema type to include SchemaWithContext and handle receiving that. (The latest versions of the wallets supports sending the schema as an object)

Changes

Expand type for schema on sendTransaction and handle the different types.

Checklist

• [x] My code follows the style of this project.
• [x] The code compiles without warnings.
• [x] I have performed a self-review of the changes.
• [x] I have documented my code, in particular the intent of the hard-to-understand areas.
• [x] (If necessary) I have updated the CHANGELOG.

[bisgardo]

I find the proposed interface hard to understand and am therefore suggesting to do it like in #34 instead.