Separate time functionality into dedicated feature flag

- Add 'time' feature flag to allow disabling time-dependent functionality
- Include 'time' in default features
- Allow users to disable SystemTime::now without disabling all std features
- Follows pattern established in other crates (e.g., lightning-transaction-sync)
- Improves compatibility with WASM environments

Author: martinsaposnic

lightning-liquidity/Cargo.toml

@@ -14,8 +14,9 @@ categories = ["cryptography::cryptocurrencies"]
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [features]
-default = ["std"]
+default = ["std", "time"]
 std = ["lightning/std"]
+time = ["std"]
 backtrace = ["dep:backtrace"]
 
 [dependencies]