Overwrite the default DLT context through dotenv

[lilianmoraru]

DLT requires a DLT context to be registered before starting to log messages.
We will offer functions to the user to register the DLT context, but to make the library easily swappable with env_logger, we will choose a default context RUST(there is a limit on the number of characters and this fits well and seems reasonable).

The idea is to offer the user the possibility to choose another default context through .env(dotenv crate) and a variable called something like: DLT_DEFAULT_CONTEXT.
Of course, the user doesn't have to use .env, he could also just export this variable before starting a build.