Set up Nethermind for the merge

[tropicar]

Is your feature request related to a problem? Please describe.

The merge will happen in a few months, every Execution clients requires some config options. This PR is to add these options to the mainnet package.

Describe the solution you'd like

Add all the configs required for the merge, All these config options must be tested on:

• Ropsten testnet
• Goerli / Prater

Detailed Changes required

• [x] OVERRIDE_TTD: X value(this will be announce in the future) Create this env var in docker compose (This value must be changeable by the user)
• [x] --Merge.TerminalTotalDifficulty=$OVERRIDE_TTD
• [x] Enable --JsonRpc.Enabled true
• [x] Create a jwtsecret file
• [x] --JsonRpc.JwtSecretFile=jwtsecret

Test results

• Ropsten
• Goerli/Prater