Allow for trusted producers for nodeos

[wanderingbort]

Propagation delay has become a problem on networks and with modern producers running entire networks of nodes to provide resilience and high availability the number of hops a block must traverse to even leave the trusted network may be > 1.

As a result, we fielded a request from BPs to add the ability to implicitly trust blocks from a given producer.

This would degrade validation for those blocks to light for any block signed by an implicitly trusted producer.

The intended use case is that a logical producer may want to implicitly trust their own blocks throughout their whole network of nodes in order to facilitate faster propagation/validation.

[brianjohnson5972]

We want to have a configuration parameter to indicate a trusted-producer, and once we validate that producer's signature for a block, then we will behave exactly like my->conf.block_validation_mode == validation_mode::LIGHT (until we are done with that block).

[arhag]

Resolved by #5631.