west: blobs: fetch `--allow-regex` filter

When building in CI for specific SOCs, it's useful to only have `west blobs
fetch` pull a selected set of blob objects. This is especially helpful on
`hal_espressif`, which currently has 78 blob objects.

Add a `--allow-regex` arg to the `west blobs fetch` subcommand to filter
only specified blobs, for example:

```bash
# only download esp32 blobs, skip the other variants
❯ west blobs fetch hal_espressif --allow-regex 'lib/esp32/.*'
```

Also, replace all `str.format()` invocations with f-strings per review
feedback.

Signed-off-by: Noah Pendleton <noah.pendleton@gmail.com>