Allow basic auth username to be explicitly defined

[msqr]

Currently when basic authorization is used because the PASSWORD environment variable is defined, the username is assumed to be the charge point ID (the CP_ID environment variable). This change is to allow an explicit basic authorization username to be used instead, by defining a CP_USERNAME environment variable. For example:

WS_URL=ws://localhost:9085/ocpp/j/v16 CP_ID=CP0 CP_USERNAME=foo PASSWORD=secret npx ts-node index_16.ts

In the case that PASSWORD is defined but CP_USERNAME is not, then the current behavior is preserved and CP_ID is used for the basic authorization username.