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/subnet/api-key.go

@@ -18,10 +18,9 @@
 package subnet
 
 import (
-	"net/url"
-
 	"github.com/minio/minio/internal/config"
 	"github.com/minio/pkg/env"
+	xnet "github.com/minio/pkg/net"
 )
 
 // DefaultKVS - default KV config for subnet settings
@@ -49,7 +48,7 @@ type Config struct {
 	APIKey string `json:"api_key"`
 
 	// The HTTP(S) proxy URL to use for connecting to SUBNET
-	Proxy string `json:"proxy"`
+	ProxyURL *xnet.URL `json:"proxy_url"`
 }
 
 // LookupConfig - lookup config and override with valid environment settings if any.
@@ -58,10 +57,12 @@ func LookupConfig(kvs config.KVS) (cfg Config, err error) {
 		return cfg, err
 	}
 
-	cfg.Proxy = env.Get(config.EnvMinIOSubnetProxy, kvs.Get(config.Proxy))
-	_, err = url.Parse(cfg.Proxy)
-	if err != nil {
-		return cfg, err
+	proxy := env.Get(config.EnvMinIOSubnetProxy, kvs.Get(config.Proxy))
+	if len(proxy) > 0 {
+		cfg.ProxyURL, err = xnet.ParseHTTPURL(proxy)
+		if err != nil {
+			return cfg, err
+		}
 	}
 
 	cfg.License = env.Get(config.EnvMinIOSubnetLicense, kvs.Get(config.License))