Pass custom network config and beacon options

[y0sher]

Description

We want to be able to pass in the node custom network config parameters that override the hardcoded network config, this is mainly for testing and local testnet scenarios.

Examples:

• running holesky-stage networkconfig but editing the AlanForkEpoch property.
• Passing in custom beacon configs like genesis time ( local testnet )

Work has already begun by @nkryuchkov here #1308

Task

• update Override spec beacon config #1308 to be aligned on stage
• make sure scenarios described above are possible
• test