Implement default jwt-secret paths

[MarekM25]

Relevant improvement in UX for Ethereum users: ethereum/execution-apis#297

Current behavior:
Nethermind is loading JwtSecret based on this config:
https://github.com/NethermindEth/nethermind/blob/master/src/Nethermind/Nethermind.JsonRpc/JsonRpcConfig.cs#L40
If the file is not found Nethermind will generate JwtSecret
Logic of loading is here: https://github.com/NethermindEth/nethermind/blob/master/src/Nethermind/Nethermind.Core/Authentication/JwtAuthentication.cs#L42
The current default value is: keystore/jwt-secret

Behavior after the change:
If JsonRpc.JwtSecretFile is not set to the default value - no change in behavior. Try to load and generate if not found from the given path.

If JsonRpc.JwtSecretFile is not specified

1. Try to load from the path specified in the proposal

For Linux:
- $XDG_DATA_DIR/ethereum/engine/jwt.hex
- $HOME/.local/share/ethereum/engine/jwt.hex

For Mac:
- $HOME/Library/Application Support/Ethereum/Engine/jwt.hex

For Windows:
- {FOLDERID_RoamingAppData}/Ethereum/Engine/jwt.hex

2. If we haven't found jwt file in the default path for OS. Try to load from the old Nethermind default.
3. If we haven't found jwt in 1. and 2. generate jwt secret file with paths specified in point 1.