Rust SDK: add encoding for unsigned operations

What

This MR adds a structure and its encoding to the Rust SDK to match the byte encoding of unsigned operations.

Closes SDK-97

Why

The objective is to provide a simple method for signing operations. This encoding will enable operations to be encoded like in octez before signing.

How

This MR adds two structs, OperationContentList and UnsignedOperation and derives their encoding using the NomReader and BinWriter deriving.

Manually testing the MR

make -C sdk/rust

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR