`load` should accept a string for settings

[ElykDeer]

When we moved the loading process for BinaryView's into the core (https://github.com/Vector35/binaryninja-api/commit/04bc6f11ae0289aae57d63d4cd32f4ef305d1a7a), we made it more difficult to set load settings by requiring a Metadata object. Several things are weird about this....one of which being that it probably should've been a Settings object. Beyond that, though, the public APIs should accept a simple string we can deserialize in the core.

This can be split into two tasks here:

• [ ] Rust/Python/C++ APIs load APIs accept a settings string
• [ ] Consider changing the core APIs just to take a string, or consider changing the Metadata object to be an actual Settings object...the original diff (https://github.com/Vector35/binaryninja-api/commit/04bc6f11ae0289aae57d63d4cd32f4ef305d1a7a) probably has some code we can bring back