Add getBlockTime rpc api (#7130)

* Add getBlockTime rpc api

* Add getBlockTime to docs

* Fix duration rounding for common tick/slot durations; add slot duration calculation

* Expose slots_per_year

* Use genesis values instead of clock defaults to calculate block offset

* Add get-block-time cli subcommand

* Fix test_rent: decrease magic number usage

sdk/src/poh_config.rs

@@ -28,6 +28,8 @@ impl PohConfig {
 
 impl Default for PohConfig {
     fn default() -> Self {
-        Self::new_sleep(Duration::from_millis(1000 / DEFAULT_TICKS_PER_SECOND))
+        Self::new_sleep(Duration::from_micros(
+            1000 * 1000 / DEFAULT_TICKS_PER_SECOND,
+        ))
     }
 }