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/clock.rs

@@ -85,6 +85,10 @@ pub type Segment = u64;
 ///  some number of Slots.
 pub type Epoch = u64;
 
+/// UnixTimestamp is an approximate measure of real-world time,
+/// expressed as Unix time (ie. seconds since the Unix epoch)
+pub type UnixTimestamp = i64;
+
 /// Clock represents network time.  Members of Clock start from 0 upon
 ///  network boot.  The best way to map Clock to wallclock time is to use
 ///  current Slot, as Epochs vary in duration (they start short and grow