adds nix flake and rust feature flags. bumps solana-sdk

[hxrts]

touches quite a few files to add feature flags, but main addition is the flake.nix

[gemini-code-assist]

Summary of Changes

Hello @hxrts, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the project's development and build infrastructure by integrating a Nix flake, providing a reproducible environment and granular control over Rust feature compilation. It also updates core Solana dependencies and refines existing feature flag usage across the codebase, ensuring more modular and efficient builds.

Highlights

• Nix Flake Integration: Introduced a Nix flake (flake.nix, flake.lock, nix/devshell.nix) to provide a reproducible development environment and build system. This setup leverages crate2nix for managing Rust dependencies within Nix, defining common build inputs, environment variables, and crate overrides for various system libraries (OpenSSL, Protobuf, SQLite, RocksDB).
• Rust Feature Flag Granularity: Refined the Rust feature flags in Cargo.toml by changing the default feature set to test-utils only, making cosmos, evm, and solana features explicitly opt-in. A new coprocessor feature was added to conditionally include the valence-coprocessor-domain-prover dependency. Numerous #[cfg(feature = "...")] attributes were added across the codebase to ensure modules and code snippets are only compiled when their respective features are enabled.
• Solana SDK Update and Mnemonic Support: Updated solana-sdk and related Solana dependencies from version 2.0 to 2.1.6. As a temporary measure, mnemonic support in SolanaClient::from_mnemonic has been disabled due to reported BIP32 compatibility issues with the new SDK version. Minor adjustments were also made to Solana RPC client methods for epoch info retrieval and transaction confirmation.
• Dependency Import Refinement: Updated several Rust files to directly import async_trait::async_trait instead of relying on tonic::async_trait, indicating a more direct and potentially cleaner dependency management practice.

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[gemini-code-assist]

Code Review

This pull request introduces a Nix flake for development environments, feature flags, and upgrades the Solana SDK. The changes are well-structured. The Solana from_mnemonic function has been disabled, which is a breaking change, and the new Nix configuration is missing the coprocessor feature in its package definitions and developer shell help text.