Dynamically create image sprites for MapLibre rendering, given a
directory with images.
### TODO
* [x] Work with @flother to merge these PRs
* [x] https://github.com/flother/spreet/pull/59 (must have)
* [x] https://github.com/flother/spreet/pull/57
* [x] https://github.com/flother/spreet/pull/56
* [ ] https://github.com/flother/spreet/pull/62 (not required but nice
to have, can upgrade later without any code changes)
* [x] Add docs to the book
* [x] Add CLI param, e.g. `--sprite <dir_path>`
* [x] Don't output `.sprites` in auto-genned config when not in use
### API
Per [MapLibre sprites
API](https://maplibre.org/maplibre-style-spec/sprite/), we need to
support the following:
* `/sprite/<sprite_id>.json` metadata about the sprite file - all coming
from a single directory
* `/sprite/<sprite_id>.png` all images combined into a single PNG
* `/sprite/<sprite_id>@2x.json` same but for high DPI devices
* `/sprite/<sprite_id>@2x.png`
Multiple sprite_id values can be combined into one sprite with the same
pattern as for tile joining:
`/sprite/<sprite_id1>,<sprite_id2>,...,<sprite_idN>[.json|.png|@2x.json|@2x.png]`.
No ID renaming is done, so identical names will override one another.
### Configuration
[Config file](https://maplibre.org/martin/config-file.html) and possibly
CLI should have a simple option to serve sprites. The configuration may
look similar to how mbtiles and pmtiles are configured:
```yaml
# Publish sprite images
sprites:
paths:
# scan this whole dir, matching all image files, and publishing it as "my_images" sprite source
- /path/to/my_images
sources:
# named source matching source name to a directory
my_sprites: /path/to/some_dir
```
Implement #705
