updates to Flurl.Http.Signed 3.2.0

[maxreb]

fixes #55
this will also update from .net standard 1.4 to .net standard 2.0 and to .net461 as flurl 3.2.0 requires it.
It contains also a docker-compose file for automatic testing

[psavva]

@lvermeulen Any chance for you to review these PR's and accept them into the main repo?
There are plenty of people who would love to help you maintain and contribute to this library as it's very useful.

[xavierpena]

I think this pull request is my only option to be able to use Keycloak.Net in my project.

My error is:

Could not load type 'Flurl.StringExtensions' from assembly 'Flurl, Version=3.0.6.0, Culture=neutral, PublicKeyToken=null'.

My architecture:

• MainProject references ProjectA and ProjectB
• ProjectA references KeyCloak.Net
• ProjectB references the last version of Flurl.Http (3.2.4)

MainProject
  --> ProjectA --> KeyCloak.Net
  --> ProjectB --> Flurl.Http v3.2.4

My only option for the moment seems to be downgrading Flurl.Http from ProjectB down to version 2.4.2, which in my case has breaking changes...