feat(charge): Add standalone charge CRUD endpoints

[rsempe]

Add charge, fixed charge, and charge filter CRUD endpoints for both plans and subscriptions to the Rust client.

[sarkissianraffi]

Hey @rsempe — I rebased this branch onto current main (just the 429-retry commit on top of the fork point). No source conflicts; Git auto-merged the lago-client/Cargo.toml change because your dep line and main's version bump touched different lines. Cargo.lock updated for the newer deps.

Rebased branch pushed as revive-charge-endpoints (didn't force-push over your branch). Local verification:

• cargo build --workspace --all-targets ✅
• cargo clippy -p lago-types -p lago-client --lib --tests -- -D warnings ✅
• cargo test --workspace --lib ✅ (23 tests pass)

Feel free to:

• Fast-forward your branch to it: git fetch origin && git reset --hard origin/revive-charge-endpoints && git push --force-with-lease, or
• Cherry-pick / ignore — your call.

Context: I needed update_plan_charge to stop losing charge lago_ids on plan updates and noticed this PR already solves it. Happy to help push it over the line.

[annvelents]

LGTM! 🚀