WeightedRoundRobin load balancer

[juliens]

What does this PR do?

Add a new load balancer to make weighted round-robin load balancing.
Weighted RoundRobin balanced on services and not on servers.

Motivation

Be able to do weighted round-robin load balancing (like canary).

More

• Added/updated tests
• Added/updated documentation

Additional Notes

Breaking changes

To be homogeneous with WRR, we renamed stickyness on server loadBalancer to sticky.

# BEFORE
- "traefik.http.services.service0.loadbalancer.stickiness=true"
- "traefik.http.services.service0.loadbalancer.stickiness.cookiename=foobar"
- "traefik.http.services.service0.loadbalancer.stickiness.httponlycookie=true"
- "traefik.http.services.service0.loadbalancer.stickiness.securecookie=true" 

# AFTER
- "traefik.http.services.service0.loadbalancer.sticky=true"
- "traefik.http.services.service0.loadbalancer.sticky.cookie.name=foobar"
- "traefik.http.services.service0.loadbalancer.sticky.cookie.httponly=true"
- "traefik.http.services.service0.loadbalancer.sticky.cookie.secure=true"

# BEFORE
[http.services.Service0.loadBalancer.stickiness]	        
  cookieName = "foobar"
  secureCookie = true
  httpOnlyCookie = true

# AFTER
[http.services.Service0.loadBalancer.sticky.cookie]
  name = "foobar"
  secure = true
  httpOnly = true

Co-authored-by: Ludovic Fernandez ldez@users.noreply.github.com

[ldez]

LGTM

[mmatur]

LGTM