Support for boolean options as flags (instead of string True/False values)

[aguschin]

Error with --current_env in mlem build venv
The docs indicate the option is boolean:
--current_env: Whether to install in the current virtual env, must be active [default: False]
but:

$ mlem build venv --model lyrics2emoji --current_env
╭─ Error ──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
│ Option '--current_env' requires an argument.                                                                                                         │
╰──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
$ mlem build venv --model lyrics2emoji --current_env True
⏳️ Loading model from lyrics2emoji.mlem
💼 Detected the virtual env /Users/aguschin/Git/iterative/mlem/emoji/.venv
💼 Installing the required packages...
......

[mike0sv]

For this to work we need to check that field that is the source of cli option has boolean type and add is_flag when we generate the option somewhere here

Need to be careful though because we also might need to alter option name. Eg if is_on has default value False, it makes sense that --is_on will make it True. But if is_on default value is True, we should have something like --not_is_on as an option name