make path of server binary configurable

[gvz]

This pull request makes the path to the Go server binary configurable. If setting argument 'bin' is not 'nil' the compilation is skipped and the value of 'bin' is used to execute the Go server from.

I needed this to make it easier to use this plugin in my nix system, as the directory of the plugin is readonly in nix system the compilation fails.

Let me know whether you want met to change something.

Btw: Thanks for the work on this great plugin.

[harrisoncramer]

Please target the develop branch with this PR, see: https://github.com/harrisoncramer/gitlab.nvim/blob/main/.github/CONTRIBUTING.md

I'm fine to merge this in as long as it doesn't impact regular usage.