[Refactor] Store values on .env file

[jmoreira-valory]

The run_service.sh script stores a number of variables as text files (e.g., rpc.txt). Instead, store these values within the .env files, unless they are not explicitly required in a file (e.g., the file agent_pkey.txt is required in a separate file, even though it is just the value of a private key).

The following approach should be used:

• Implement a method set_env_var VAR VALUE which:
  • Creates the .env file if it does not exist.
  • Updates VAR=OLD_VALUE to VAR=VALUE if the entry for VAR exists in .env file.
  • Appends VAR=VALUE to the env file if VAR does not exist in the .env file.
  • exports VAR=VALUE to the environment to be used in the rest of the script.

The file .env should be sourced at the appropriate point at the start of the script.

#99 (comment)

[jmoreira-valory]

Partialy completed on #111 : The method is completed there, and used for USE_STAKING and AGENT_ID; the rest of the eligible variables are not addressed.