HBASE-25896 Implement a Region Visualization on Master WebUI (#4228)

HBASE-25896 Implement a Region Visualization on Master WebUI (#4228)

This is a demonstration of visualization of regions on the cluster. The visualization is a stacked
bar chart showing total storefile size per table per region server, with the x-axis being server
names, the y-axis being storfile size, and the bars stacked per table. The visualization is
generated entirely on the fly from within the browser, implemented using Vega Lite. So far, Vega
appears to handle rendering this visualization for a cluster of over 700 region servers with
approximately 300,000 regions.

Per [0], include an update to the top-level LICENSE.txt. Also update LICENSE files in all binary
distributions (i.e., jars), by way of LICENSE.vm. Vega uses a BSD 3-clause variant without
advertising clause, and as such is a "Category A" license, per [1].

No changes are made to the NOTICE files, as per the existing example of bundling the minified
JQuery, which is also a Category A license.

[0]: https://infra.apache.org/licensing-howto.html
[1]: https://www.apache.org/legal/resolved.html#category-a

Signed-off-by: Andrew Purtell <apurtell@apache.org>