Extra attributes should be forbidden in nested Input models

[carl-baillargeon]

Nested input models like class BgpAfi(BaseModel) don't inherit from AntaTest.Input meaning they don't forbid extra attributes. This could lead to unexpected behaviors for a user who wants to override a default value but accidently provides the wrong key.

BGP tests for example. If a user provides vrfs instead of vrf, ANTA will silently ignore vrfs and use the default value of vrf. An error should be raised to the user since vrfs is not supported.

[carl-baillargeon]

Nested input models are being moved over to anta.input_models package (see #388) and model_config = ConfigDict(extra="forbid") should be added at the same time.