Support for reading ScVal contract parameters and fuzzing over them

[gtrepta]

This implements a number of the available SCVals in soroban as dataclasses that allow reading of the bindings from a soroban contract and generating values for them with hypothesis.

[bbyalcinkaya]

Great work on the code! Everything looks good.

Just FYI, if we want to support custom types in bindings in the future, we'll need to find a way to distinguish between structs and tuples (haven't tried with enums) in the bindings JSON generated by the soroban CLI. Right now, we can get away without it since we only need bindings for test contracts. Structs and tuples look the same in the JSON, but they should be encoded differently (Vec for tuples and Map for structs).

For example:

#[contracttype]
pub struct Foo(u32, u32);

#[contracttype]
pub struct Bar {
  x: u32,
  y: u32
}

#[contractimpl]
impl TypesContract {
    pub fn struct_tuple(_env: Env, _tuple: Foo, _struct: Bar) { }
}

The generated JSON for the struct_tuple endpoint using these types will not differentiate between Foo (tuple) and Bar (struct):

{
  "type": "function",
  "doc": "",
  "name": "struct_tuple",
  "inputs": [
    {
      "doc": "",
      "name": "_tuple",
      "value": {
        "type": "custom",
        "name": "Foo"
      }
    },
    {
      "doc": "",
      "name": "_struct",
      "value": {
        "type": "custom",
        "name": "Bar"
      }
    }
  ],
  "outputs": []
}

If we ever decide to support this, we'll need a solution for distinguishing these types.