Bug 1967925

I'm thinking about how to achieve the required functionality with the current DNF API.

A possible solution:
dnf.repo.Repo object allows to get/set repository configuration options using getattr/setattr.

Example of a function using the existing DNF API:
```
def repo_set_opt(dnf_repo_object, option_name, option_value):
    if hasattr(dnf_repo_object, option_name):
        setattr(dnf_repo_object, option_name, option_value)
    else:
        raise Exception('Unknown option: {}'.format(option_name))
```

Example of use (description of result in comment):
repo_set_opt(repo, 'metadata_expire', '4d') # sets `metadata_expire` to 4 days

repo_set_opt(repo, 'metadata_expire', '4dr') # dnf.exceptions.ConfigError: Error parsing '4dr': could not convert '4dr' to seconds

repo_set_opt(repo, 'm_expire', '4d') # Exception: Unknown option: m_expire

repo_set_opt(repo, 'enabled', '1') # Sets boolean value `enabled` to True

repo_set_opt(repo, 'enabled', 'ewe') # dnf.exceptions.ConfigError: Error parsing 'ewe': invalid boolean value 'ewe'

repo_set_opt(repo, 'excludepkgs', 'pkg1,pkg2') # sets `excludepkgs` list
print(getattr(repo,'excludepkgs')) # test of previous action, prints list "['pkg1', 'pkg2']"

Note:
The same parser (rules for parsing values) is used as for the `--setopt` DNF argument and configuration values in the files.

`setattr` is a very powerful function. I give a safer example. Added test to prevent object methods from being overwritten.

```
def repo_set_opt(dnf_repo_object, option_name, option_value):
    if hasattr(dnf_repo_object, option_name) and not callable(getattr(dnf_repo_object, option_name)):
        setattr(dnf_repo_object, option_name, option_value)
    else:
        raise Exception('Unknown option: {}'.format(option_name))
```

It is still not a perfect solution. But it could be enough.

As I wrote, `setattr` is still not perfect. E.g. "append" type options (eg "excludepkgs") are handled as general (not append) options.
So, I created an API for setting configuration options using string arguments. The API also works for setting global configuration options, not just repositories.

https://github.com/rpm-software-management/dnf/pull/1779