Refactor Data Pipeline

[nikmo33]

From a conversation with @ethanweber decided to take https://github.com/nerfstudio-project/nerfstudio/pull/886 a step further and cleanup input metadata overall and make this pipeline more generic. Adds input metadata that may be useful for adding new input modalities in the future. Left mask as a separate variable from metadata as it is one that is probably something we want to support in the base dataset? Added an example usage with semantic nerfw and associated data code

Move datamanagers into their own folder and split into base/semantic datamanagers Move datasets into their own folder and split into base/semantic datasets

Have tested with vanilla nerfacto with poster dataset and semantic-nerfw with friends-data. Also checked viewer and everything seems to be working there + ns-dev-test