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

client/src/rpc_request.rs

@@ -111,6 +111,7 @@ pub enum RpcRequest {
     ValidatorExit,
     GetAccountInfo,
     GetBalance,
+    GetBlockTime,
     GetClusterNodes,
     GetEpochInfo,
     GetEpochSchedule,
@@ -150,6 +151,7 @@ impl RpcRequest {
             RpcRequest::ValidatorExit => "validatorExit",
             RpcRequest::GetAccountInfo => "getAccountInfo",
             RpcRequest::GetBalance => "getBalance",
+            RpcRequest::GetBlockTime => "getBlockTime",
             RpcRequest::GetClusterNodes => "getClusterNodes",
             RpcRequest::GetEpochInfo => "getEpochInfo",
             RpcRequest::GetEpochSchedule => "getEpochSchedule",