Improve payload_attributes event frequency

[zilm13]

Description

One of the goals of introducing payload_attributes event was to help MEV builders to start constructing new payload as early as possible. Builder could be called on any slot so such systems needs new payload_attributes on every fcu (see comment). Following line was added to the spec:

The frequency at which this event is sent may depend on beacon node configuration.

Our current implementations sends payload_attributes event only when beacon node is called to produce block, so it cannot be used for similar job. We could enhance our imlementation to send payload_attributes event on every fcu, so it could be used for such cases. Calculating all fields is not free, so it makes sense to introduce command line options flag which will turn this feature on, so by default we will keep sending event only in our slots when we have proposal.