Provide a TensorBoard-native summary writer API

This issue tracks work to provide, within TensorBoard proper, a summary "writer" API for end-users to log data for visualization in TensorBoard. This is intended to be a lightweight way to log to TensorBoard without necessarily depending on TensorFlow, although it should also be usable by eager-mode TensorFlow users.

Note that this API isn't intended to replace tf.summary - the latter will continue to be the recommended approach for TensorFlow users who want to log data from within graphs or tf.functions.

Plan of work. Some of these steps may be done on-demand / as need arises.

• Define initial Writer object and Output interface with scalar support
• Port tf.summary summary types
  • Scalars
  • Images
  • Audio
  • Histograms
  • Text
  • HParams
  • Graph
  • PR curves
  • Custom scalars
  • Mesh
  • Embedding
  • NPMI
• Add support for generic / extensible summary types
• Add support for tracking a step within the writer itself
• Add Output mechanism for logging the version of TensorBoard used to write data
• Implement other Output types
  • In-memory output
  • Logging to stdout
• Implement filesystem support for DirectoryOutput
  • Local filesystem
  • tf.io.gfile support
  • GCS support (without TF)
• Add API documentation
• Add demo (or retrofit existing demos)

No files, tests, or entry points are identified in the issue. Begin by narrowing the planned Writer and Output interfaces, then determine the initial scope and acceptance criteria before implementing the listed summary, filesystem, documentation, and demo work.