Basic auth for handlers

[aram356]

Closes #73

Example configuration

1. in trusted-server.toml

[[handlers]]
path = "^/secure"
username = "user"
password = "pass"

2. Environment variables:

# [[handlers]]
TRUSTED_SERVER__HANDLERS__0__PATH=^/secure
TRUSTED_SERVER__HANDLERS__0__USERNAME=user
TRUSTED_SERVER__HANDLERS__0__PASSWORD=pass

[ChristianPavilonis]

👍