Support specify a directory instead of a single file which the argument is `-f` when generate models

[LeoLiuYan]

Feature Request

Is your feature request related to a problem? Please describe:

Describe the feature you'd like:
When generate KCL models, we expect to use a directory argument that all source OpenAPI files in it.
And the command of kcl-openapi generate model should scan all files in the directory.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[Peefy]

Supported in the KCL new cli kcl import command. Ref: https://github.com/kcl-lang/cli/blob/main/pkg/options/import.go#L38