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/callhome/help.go

@@ -0,0 +1,42 @@
+// Copyright (c) 2015-2022 MinIO, Inc.
+//
+// This file is part of MinIO Object Storage stack
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package callhome
+
+import "github.com/minio/minio/internal/config"
+
+var (
+	defaultHelpPostfix = func(key string) string {
+		return config.DefaultHelpPostfix(DefaultKVS, key)
+	}
+
+	// HelpCallhome - provides help for callhome config
+	HelpCallhome = config.HelpKVS{
+		config.HelpKV{
+			Key:         Enable,
+			Type:        "on|off",
+			Description: "set to enable callhome" + defaultHelpPostfix(Enable),
+			Optional:    true,
+		},
+		config.HelpKV{
+			Key:         Frequency,
+			Type:        "duration",
+			Description: "time duration between callhome cycles e.g. 24h" + defaultHelpPostfix(Frequency),
+			Optional:    true,
+		},
+	}
+)