Add LLM Agent example. (#3150)

## Motivation

Linera's architecture offers a unique opportunity for AI agents to
interact with blockchains with low-latency and at low cost by offering
them their own blockspace in microchains.

## Proposal

This example application shows how this can be achieved using the `rig`
framework and a simple CLI REPL.

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

## Links

https://rig.rs/

Author: christos-h

.github/workflows/rust.yml

@@ -315,6 +315,8 @@ jobs:
       run: |
         cd examples
         cargo sort --check --workspace --grouped
+        cd agent
+        cargo sort --check
 
   lint-check-for-outdated-readme:
     runs-on: ubuntu-latest
@@ -353,6 +355,8 @@ jobs:
         cargo fmt -- --check
         cargo clippy --all-targets --all-features --target wasm32-unknown-unknown --locked
         cargo clippy --all-targets --all-features --target x86_64-unknown-linux-gnu --locked
+        cd agent
+        cargo fmt -- --check
 
   lint-cargo-clippy:
     runs-on: ubuntu-latest