CP-308049: rrdview tool (#6468)

I wrote this in ~2022, so I don't fully remember how it all works, but I
tried to document what I know in the commit message, in the CLI flag
docs and here:
```
scp -r root@$YOURBOX:/var/lib/xcp/blobs/rrds /tmp/rrds
dune exec ./rrdview.exe -- /tmp/rrds
bash /tmp/rrds/16db833b-7cd6-4b69-9037-144076c71033.cpu_avg.DERIVE.sh
```

![rrd](https://github.com/user-attachments/assets/b3c98936-cc29-4871-b4ea-1511853d623e)

Viewing RRDs produced by xcp-rrdd is difficult, because the format is
incompatible with rrdtool. rrdtool has a hardcoded limit of 20 char for
RRD names for backward compat with its binary format.

Steps:
* given a directory of xml .gz files containing xcp-rrdd produced rrds
* invokes itself recursively with each file in turn using xargs -P (easy
way to parallelize on OCaml 4)
* load all RRDs, and split them into separate files, allowing us to
shorten many of their names without conflicts
* some names are still too long, there is a builtin translation table to
shorten these
* once split an .rrd file is created using 'rrdtool restore'. This can
further be queried/inspected/transformed by rrdtool as needed
* a .sh script is produced that can plot the RRD if desired.

There are many RRDs so plotting isn't done automatically yet.

RRDs contain min/avg/max usually, so this is drawn as a strong line at
the average, and an area in a lighter color for min/max (especially
useful for historic data that has been aggregated).

Caveats:
* we don't know the unit name, that is part of the XAPI metadata, but
not the XML apparently?
* separate plots are generated for separate intervals, it'd be nice to
join all these into the same graph
* the visualization type is not the best for all RRDs, some might
benefit from a smoother line, etc.
* for now the tool is just built, but not installed (that'll require a
.spec change too and can be done later)
* there is some code there to start parsing the data source definitions,
eventually I wanted to plot the data using OCaml instead of rrdtool
(e.g. generate Vega/Vega-lite graphs), but I can't find which branch I
put *that* code on, what I have here is incomplete (or maybe I never
wrote that part, just thought about it). We could trim the dead code
from here if needed, but it might be useful if we continue improving the
tool later, so for now I left the parsing in.

This is just a starting point to be able to visualize this data somehow,
and we can improve the actual plotting later.

Author: edwintorok