Allow multiple download or auth plugins for a single provider

[sbrunato]

Allow multiple download or auth plugins for a single provider:

!provider
  name: foo_provider
  search: !plugin
    type: StacSearch
    [...]
  download:
    - !plugin
      type: HTTPDownload
      match: 
        href: https://[-\w]+.foo.bar
    - !plugin
      type: AwsDownload
      match: 
        href: s3://
  auth:
    - !plugin
      type: GenericAuth
      match: 
        roles:
          - archive
    - !plugin
      type: AwsAuth
      match: 
        href: s3://

Use matching_url to choose appropriate plugin when using EOProduct.register_downloader method.

This feature will be useful when Asset.register_downloader is available.