[solver/protocol-smart-contracts] Integrate SaveResult to Solver

[narbs91]

General Description

As a developer I want to add the SaveResult method to the apiv2.go module (part of #529) so that the solver can utilize this method to save result on-chain

Which system(s) or functionality does this affect

This can be one or many. Please also include this in the title

solver, protocol-smart-contracts

Describe the changes, and how this affects/ interacts with each system.

1. Add the SaveResult method to the apiv2 module
2. Update the solver code on job completion/failure to call the SaveResult
3. Update corresponding logic in the anura/job_offers/{id}/files endpoint
4. Ensure the solver code change is gated by the lilynext feature flag