RFC: Modular Filesystems C API (#101)

* Propose an RFC for filesystem plugins under modular TensorFlow

* Update path to image files

The RFC files got moved to a new directory but text was still using the old path.

* Add mention of cloud environments

* Expand compatibility defintion/requirements

* Better time based qualifiers

* Specify that scheme uniquely identifies filesystem implementation

* Separate higher level APIs into appendix

* Specify that new method will call the old one when only signature changes

* Specify that versioning information is ellided when presenting API

* Env class can be just extended with a new lookup layer instead of subclassing

* Add questions, cost and leaning

* Fix appendix link

* Fix spelling of elided

* Remove duplicated character typo

* Remove needless capitalization

* Change status to accepted

Author: mihaimaruseac