Add .mbtiles support (#549)

Adds a new [.mbtiles](https://github.com/mapbox/mbtiles-spec/blob/master/1.3/spec.md)
backend, without the grid support. Uses extensive tile content
detection, i.e. if the content is gzipped, png, jpeg, gif, webp.

From CLI, can be as easy as adding a path to a directory that contains a
.mbtiles file (works just like pmtiles support)

```bash
# All *.mbtiles files in this dir will be published.
# The filename will be used as the source ID
martin ./tests/fixtures
```

From configuration file, the path can be specified in a number of ways
(same as pmtiles)

```yaml
mbtiles:
  paths:
    # scan this whole dir, matching all *.mbtiles files
    - /dir-path
    # specific mbtiles file will be published as mbtiles2 source
    - /path/to/mbtiles2.mbtiles
  sources:
      # named source matching source name to a single file
      pm-src1: /tmp/mbtiles.mbtiles
      # named source, where the filename is explicitly set. This way we will be able to add more options later
      pm-src2:
        path: /tmp/mbtiles.mbtiles
```

Fixes #494

.cargo/config.toml

@@ -0,0 +1,3 @@
+[alias]
+installsqlx = "install sqlx-cli --no-default-features --features sqlite,native-tls"
+sqlxprep = "sqlx prepare --merged --database-url sqlite://mbtiles/data/geography-class-jpg.mbtiles"