feat: add intervalSeed source option

* adds an `intervalSeed` source option
  * enables configuration of deterministic offset to polling
  * disabled by default (no offset, wall-clock polling is maintained
* improves sync source logging consistency

ref #1945

Signed-off-by: Todd Baert <todd.baert@dynatrace.com>

Author: toddbaert

docs/reference/flagd-cli/flagd_start.md

@@ -31,7 +31,7 @@ flagd start [flags]
   -c, --server-cert-path string              Server side tls certificate path
   -k, --server-key-path string               Server side tls key path
   -d, --socket-path string                   Flagd unix socket path. With grpc the evaluations service will become available on this address. With http(s) the grpc-gateway proxy will use this address internally.
-  -s, --sources string                       JSON representation of an array of SourceConfig objects. This object contains 2 required fields, uri (string) and provider (string). Documentation for this object: https://flagd.dev/reference/sync-configuration/#source-configuration
+  -s, --sources string                       JSON representation of an array of SourceConfig objects. Required fields: uri (string) and provider (string). Optional source-specific fields are also available, see https://flagd.dev/reference/sync-configuration/#source-configuration
       --stream-deadline duration             Set a server-side deadline for flagd sync and event streams (default 0, means no deadline).
   -g, --sync-port int32                      gRPC Sync port (default 8015)
   -e, --sync-socket-path string              Flagd sync service socket path. With grpc the sync service will be available on this address.

flagd/cmd/start.go

@@ -72,9 +72,10 @@ func init() {
 	)
 	flags.StringSliceP(corsFlagName, "C", []string{}, "CORS allowed origins, * will allow all origins")
 	flags.StringP(
-		sourcesFlagName, "s", "", "JSON representation of an array of SourceConfig objects. This object contains "+
-			"2 required fields, uri (string) and provider (string). Documentation for this object: "+
-			"https://flagd.dev/reference/sync-configuration/#source-configuration",
+		sourcesFlagName, "s", "", "JSON representation of an array of SourceConfig objects. "+
+			"Required fields: uri (string) and provider (string). "+
+			"Optional source-specific fields are also available, "+
+			"see https://flagd.dev/reference/sync-configuration/#source-configuration",
 	)
 	flags.StringP(logFormatFlagName, "z", "console", "Set the logging format, e.g. console or json")
 	flags.StringP(metricsExporter, "t", "", "Set the metrics exporter. Default(if unset) is Prometheus."+