implement dynamic generation of service definition files and start scripts

[rswanson]

need for #12 also improves repo experience for end users as they don't need to maintain their own files for these anymore

[rswanson]

Things I need to expose in a config object for service files:

• Binary Name (reth, lighthouse, etc)
• Linux user name
• Network (mainnet, holesky, sepolia)
• Instance Number (integer)
• Syslog identifier (could I have a default of <binary name>-<network>-<instance number> ?)
• Start script location

[rswanson]

Things I need to expose in a config object for start scripts

• Network
• Start command
• Args?
  • how do I structure args? Probably some kind of generic struct for arbitrary args?
• Instance Number
• Data dir?