Feature request: environement variable extrapolation in configuration

Hi 👋🏼 Would it be possible to add environment variables extrapolation support in the config file? Due to security, I avoid having credentials stored in a clear text file and often rely on a password manager (gopass, 1password, bitwarden...). To have some security as well as some flexibility, I would love to be able to write a profile with this kind of syntax: ```toml [profiles.my-db] adapter = "postgres" host = "{env.MY_DB_HOST}" user = "{env.MY_DB_USER}" password = "{env.MY_DB_PASSSWORD}" dbname = "{env.MY_DB_NAME}" ``` Or using pyproject.toml ```toml [tool.harlequin.profiles.my-db] adapter = "postgres" host = "{env.MY_DB_HOST}" user = "{env.MY_DB_USER}" password = "{env.MY_DB_PASSSWORD}" dbname = "{env.MY_DB_NAME}" ``` It would help with password managers but also when having DB access stored in a `.env` file, which can be automatically loaded. I don't have any preference for the syntax; it could also be `{env:VAR}` or whatever. Does it seem acceptable to you? Would you accept a pull request for it?