Cli: better estimate of epoch time elapsed/remaining (#15893) (#15918)

* Add rpc_client api for getRecentPerformanceSamples

* Prep fn for variable avg slot time

* Use recent-perf-samples to more-accurately estimate epoch completed times

* Spell out average

(cherry picked from commit 3726358f51)

Co-authored-by: Tyera Eulberg <teulberg@gmail.com>

client/src/rpc_request.rs

@@ -34,6 +34,7 @@ pub enum RpcRequest {
     GetMultipleAccounts,
     GetProgramAccounts,
     GetRecentBlockhash,
+    GetRecentPerformanceSamples,
     GetSnapshotSlot,
     GetSignatureStatuses,
     GetSlot,
@@ -90,6 +91,7 @@ impl fmt::Display for RpcRequest {
             RpcRequest::GetMultipleAccounts => "getMultipleAccounts",
             RpcRequest::GetProgramAccounts => "getProgramAccounts",
             RpcRequest::GetRecentBlockhash => "getRecentBlockhash",
+            RpcRequest::GetRecentPerformanceSamples => "getRecentPerformanceSamples",
             RpcRequest::GetSnapshotSlot => "getSnapshotSlot",
             RpcRequest::GetSignatureStatuses => "getSignatureStatuses",
             RpcRequest::GetSlot => "getSlot",