RPC: implement SerializeForVersion for RPC v0.6

[Mirko-von-Leipzig]

Requires #1830.

Once #1830 is completed, all methods should be available for use which enables switching to using these for all RPC versions.

I propose only doing this once RPC version v0.4 and v0.5 have been removed. This (a) lessens the work, and (b) is safer since some methods have changed semantically (and have different error types for example).

This leaves RPC v0.6:

• Implement existing DTO changes in the serialization code by diffing RPC v0.6 and v0.7 of the specification.
• Switch all v0.6 methods to the crates/rpc/method/ implementations
  • [ Ensure the DTO changes defined within each method itself are also implemented (these DTOs are specified within the method implementation itself).