feat(manager/gomod): inject credentials in GOPROXY environment variable

[Shegox]

Changes

This PR adds support for injecting credentils from the hostType: go-proxy into the GOPROXY enviroment variable when updating go modules.

This allows one to use Renovate to update dependencies which only exist in an private, authenticated GOPROXY and not e.g. in the backing version control system e.g. in Artifactory.

Lookups in the go datasource already work with authentication and hostType: go-proxy.

Additionally this PR adds some documentation for this.
In the tests I switched from mocking the hostRule resolutations to using the actual hostRule implementation allowing for easier to read and update tests.

Context

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• This closes an existing Issue, Closes: #
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The public repository: n/a, run on a private repository on GitHub Enterprise Server with a private JFROG artifactory