Add providerPostfix configuration

[ValentinVignal]

Closes #1851

Add the configuration to be able to have another postfix than "Provider".

[ValentinVignal]

@rrousselGit I'm creating a draft PR because I would have several questions before going further with this:

• Do you prefer "postfix" or "suffix"?
• How would you want me to write a test for that?
  I could create a "golden" test like what is done in riverpod_graph: I create a sub-package / app with its own pubspec.yaml and build.yaml (so I can modify the build options).
  I could for example copy the sync.dart file in it. But then do I also copy the sync_test and use all the renamed providers? This doesn't seem so right to me. Do you have any suggestions?

[codecov]

Codecov Report

Merging #1930 (6ca125e) into master (858872c) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1930   +/-   ##
=======================================
  Coverage   95.48%   95.48%           
=======================================
  Files          49       49           
  Lines        2062     2062           
=======================================
  Hits         1969     1969           
  Misses         93       93           

[ValentinVignal]

I didn't see there already was a PR for that ... 🤦 #1898