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>

internal/config/config.go

@@ -88,6 +88,7 @@ const (
 	ScannerSubSys        = "scanner"
 	CrawlerSubSys        = "crawler"
 	SubnetSubSys         = "subnet"
+	CallhomeSubSys       = "callhome"
 
 	// Add new constants here if you add new fields to config.
 )
@@ -161,6 +162,7 @@ var SubSystems = set.CreateStringSet(
 	NotifyRedisSubSys,
 	NotifyWebhookSubSys,
 	SubnetSubSys,
+	CallhomeSubSys,
 )
 
 // SubSystemsDynamic - all sub-systems that have dynamic config.
@@ -170,6 +172,7 @@ var SubSystemsDynamic = set.CreateStringSet(
 	ScannerSubSys,
 	HealSubSys,
 	SubnetSubSys,
+	CallhomeSubSys,
 	LoggerWebhookSubSys,
 	AuditWebhookSubSys,
 	AuditKafkaSubSys,