(new commit message)

This adds a generic mechanism in the Download API to rewrite the URL
to be downloaded, possibly with extra headers. Substitued groups from
the matching regular expression can be optionally quoted.

.netrc:

macdef buildout:HOSTNAME
  RULE_1_REGEX
    RULE_1_NEW_URL HEADER1=VALUE1 HEADER2=VALUE2 ...
  RULE_2_REGEX
    RULE_2_NEW_URL ...
  ...

macdef buildout:OTHER_HOSTNAME
  ...

A rewriting rule is defined by a pair of lines with optional indentation
(only there for readability).

The first line of a rule is a regex that matches fully against
the path?query part of the URL.

If the second line is empty, the request isn't changed.
Else parts are parsed using shell-like syntax:
- the first one must produce the full URL to download
- the next ones are optional headers to send
- each part is subject to regex substitution using the
  Python Format Specification Mini-Language:
  captured grouped from the first line are numbered starting from 1,
  {0} is the base URL (scheme://location) and
  the optional `quote` attribute returns the urlencoded value

A use case is to work around
  https://gitlab.com/gitlab-org/gitlab/-/issues/19189
for example with the following .netrc:

    macdef buildout:lab.nexedi.com
      /(.+)/-/raw/([^/]+)/(.+)
        {0}/api/v4/projects/{1.quote}/repository/files/{3.quote}/raw?ref={2} PRIVATE-TOKEN=<ACCESS_TOKEN>