This commit adds possibility to omit tenantID in the URL path. In this case,
tenantID will be fetched from HTTP headers `AccountID` and `ProjectID`.
If headers are missing too, then default `0:0` tenantID is used.
This functionality can be enabled only if -enableMultitenantHandlers
cmd-line flag was set to vminsert, vmselect or vmagent.
Motivation: this change makes VM configuration for multienancy
consistent with VL configuration - see
https://docs.victoriametrics.com/victorialogs/#multitenancy. And keeps
backward compatibility in the same time.
fixes https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4241
