Bellatrix: pass justified as a safe block

[mkalinin]

Introduces a get_safe_block_hash function that returns a block hash of a safe execution block. This function may be redefined post-Merge. The current definition uses the most recent justified block as a safe block.

An alternative approach is in having generally defined get_safe_block function (as an extension outside of the main fork-choice spec) utilized by get_safe_block_hash. But it can be easily done later on once get_safe_block will represent more complicated logic than it currently is, or I can be convinced that it should be done within this PR.

TODO

• consider get_safe_block implementation returning store.justified_checkpoint.root
• write tests (left for subsequent PR, tracked in Add minimal unit tests for safe block functions #2865)

[mkalinin]

I have added a /fork_choice/safe-block.md document with the function definition. I am free to debate on the naming and place of this small piece of the spec. Once this is done it can be included in the spec build and testing

[djrtwo]

I think this should likely got in the safe_block helper file

[mkalinin]

Applied the following review suggestions:

• get_safe_beacon_block -> get_safe_beacon_block_root
• get_safe_block_hash -> get_safe_execution_payload_hash
• get_safe_execution_payload_hash moved to safe-block.md

[hwwhww]

Note that this file is cross forks. Maybe note that it is only valid post-Bellatrix.

[mkalinin]

In general, safe block algorithm can be implemented with Phase0 structures. I would add a note that get_safe_execution_payload_hash helper is based on Bellatrix

[djrtwo]

looks good to me! opened #2865 to track needing some unit tests here