Add mainnet merge values 🐼 (#3462)

## Issue Addressed

NA

## Proposed Changes

Adds **tentative** values for the merge TTD and Bellatrix as per ethereum/consensus-specs#2969

## Additional Info

- ~~Blocked on ethereum/consensus-specs#2969

common/eth2_network_config/built_in_network_configs/mainnet/config.yaml

@@ -6,8 +6,8 @@ PRESET_BASE: 'mainnet'
 
 # Transition
 # ---------------------------------------------------------------
-# TBD, 2**256-2**10 is a placeholder
-TERMINAL_TOTAL_DIFFICULTY: 115792089237316195423570985008687907853269984665640564039457584007913129638912
+# Estimated on Sept 15, 2022
+TERMINAL_TOTAL_DIFFICULTY: 58750000000000000000000
 # By default, don't use these params
 TERMINAL_BLOCK_HASH: 0x0000000000000000000000000000000000000000000000000000000000000000
 TERMINAL_BLOCK_HASH_ACTIVATION_EPOCH: 18446744073709551615
@@ -35,7 +35,7 @@ ALTAIR_FORK_VERSION: 0x01000000
 ALTAIR_FORK_EPOCH: 74240
 # Merge
 BELLATRIX_FORK_VERSION: 0x02000000
-BELLATRIX_FORK_EPOCH: 18446744073709551615
+BELLATRIX_FORK_EPOCH: 144896 # Sept 6, 2022, 11:34:47am UTC
 # Sharding
 SHARDING_FORK_VERSION: 0x03000000
 SHARDING_FORK_EPOCH: 18446744073709551615