modules: Basic binary blob infrastructure

This patch introduces the basic infrastructure to list and fetch binary
blobs. This includes:

- The new 'blobs' extension command
- An implementation of the `west blobs list` command
  with custom formatting
- A very simple mechanism for loading fetchers
- A basic implementation of an HTTP fetcher

In order to ensure consistency among the west extension commands in the
main zephyr tree, we reuse a similar class factory pattern that is present
for ZephyrBinaryRunner instances in the ZephyrBlobFetcher case. This
could be achieved with a simpler mechanism, but opted for consistency
before simplicity.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>