Add periodic callhome functionality (#14918)

* Add periodic callhome functionality

Periodically (every 24hrs by default), fetch callhome information and
upload it to SUBNET.

New config keys under the `callhome` subsystem:

enable - Set to `on` for enabling callhome. Default `off`
frequency - Interval between callhome cycles. Default `24h`

* Improvements based on review comments

- Update `enableCallhome` safely
- Rename pctx to ctx
- Block during execution of callhome
- Store parsed proxy URL in global subnet config
- Store callhome URL(s) in constants
- Use existing global transport
- Pass auth token to subnetPostReq
- Use `config.EnableOn` instead of `"on"`

* Use atomic package instead of lock

* Use uber atomic package

* Use `Cancel` instead of `cancel`

Co-authored-by: Harshavardhana <harsha@minio.io>

Co-authored-by: Harshavardhana <harsha@minio.io>
Co-authored-by: Aditya Manthramurthy <donatello@users.noreply.github.com>

cmd/admin-server-info.go

@@ -31,7 +31,10 @@ import (
 // local endpoints from given list of endpoints
 func getLocalServerProperty(endpointServerPools EndpointServerPools, r *http.Request) madmin.ServerProperties {
 	var localEndpoints Endpoints
-	addr := r.Host
+	addr := globalLocalNodeName
+	if r != nil {
+		addr = r.Host
+	}
 	if globalIsDistErasure {
 		addr = globalLocalNodeName
 	}
@@ -40,7 +43,7 @@ func getLocalServerProperty(endpointServerPools EndpointServerPools, r *http.Req
 		for _, endpoint := range ep.Endpoints {
 			nodeName := endpoint.Host
 			if nodeName == "" {
-				nodeName = r.Host
+				nodeName = addr
 			}
 			if endpoint.IsLocal {
 				// Only proceed for local endpoints