Allow forward proxy configuration for git repositories

Issue Summary

This is reproducible on Data Center: (yes)

On plane git repository configuration, add support for forward proxy configuration and a mechanism to propagate the configuration to the remote agents.

Steps to Reproduce

1. Create a new linked repository
2. Configure git repository that is accessible only thru forward HTTP/HTTPS proxy
3. Configuration will fail

Expected Results

Bamboo UI should allow to specify eventual forward proxy configuration.

Actual Results

No forward proxy configuration available:

Workaround

1. Configure the git client that Bamboo is using as the Git capability to use a forward proxy. For example run the following commands as the same user that runs Bamboo on the Bamboo machine:

   $ git config --global http.proxy http://<username>:<password>@<forward_proxy_hostname>:<forward_proxy_port>
   $ git config --global http.proxyAuthMethod basic
   

   where you replace the <username>, <password>, <forward_proxy_hostname>, <forward_proxy_port> with the actual values. In the example above the basic authentication is used but you can find more options to configure the forward proxy related parameters at https://git-scm.com/docs/git-config The configurations would be saved under ~/.gitconfig

1. Configure the repository entry in the Bamboo Linked repository as type Git
2. Manually add the same git config to the remote agents on which the repository will be consumed.