build: fix rust toolchain to 1.79

[prasannavl]

Summary

• Add rust-toolchain.toml to fix the rust toolchain.
• Note: Currently set to 1.79.
  • Ref: https://releases.rs/docs/1.80.0/
  • https://internals.rust-lang.org/t/type-inference-breakage-in-1-80-has-not-been-handled-well/21374

Implications

• Storage

  • Database reindex required
  • Database reindex optional
  • Database reindex not required
  • None

• Consensus

  • Network upgrade required
  • Includes backward compatible changes
  • Includes consensus workarounds
  • Includes consensus refactors
  • None