Add proxy configuration at the context level

[placintaalexandru]

Fixes #825
Fixes #371

This PR adds proxy configuration (new field proxy) at the context level as seen in the config bellow:

k9s:
  cluster: kind-k9s-test
  namespace:
    active: default
    lockFavorites: false
    favorites:
    - default
  view:
    active: po
  featureGates:
    nodeShell: false
  portForwardAddress: localhost
  proxy:
    address: http://localhost:3128

Testes with a squid proxy:

http_port 80
http_port 3128

# Define ACLs
acl all_traffic src all

# Request forwarding configuration
request_header_access Authorization allow all

# Allow all traffic
http_access allow all_traffic

# Define a custom log format named "combined"
logformat squid      %ts.%03tu %6tr %>a %Ss/%03>Hs %<st %rm %ru %[un %Sh/%<a %mt

# Use the custom log format for access logging
access_log stdio:/var/log/squid/access.log squid