add ability to set computed env vars for gh cli

[mrjones2014]

Describe what this PR does / why we need it

Adds a hook to get computed environment variables when running the GitHub CLI. It is needed, for example, to allow you to set GITHUB_TOKEN from the 1Password CLI.

Does this pull request fix one issue?

Fixes #345

Describe how you did it

Added a config option that allows user to pass a function that returns a table to merge with env_vars

Describe how to verify it

require('octo').setup({
  get_env = function() return { GITHUB_TOKEN = 'your token here' } end,
})

This config should authenticate the GitHub CLI via octo.nvim

Special notes for reviews

Here's how I've configured mine, integrating with the op.nvim plugin, using its API:

require('octo').setup({
  get_env = function()
    local github_token = require('op.api').item.get({ '[item uuid here]', '--fields', 'token' })[1]
    if not github_token or not vim.startswith(github_token, 'ghp_') then
      error('Failed to get GitHub token.')
    end

    return { GITHUB_TOKEN = github_token }
  end,
})

[pwntester]

Thanks for the PR!

Although in your specific use case it makes sense for get_env to be a function, I would expect most users would suffice to use a table. Can you make the following changes?

• rename get_env to gh_env
• handle both function and table cases
• document the new gh_env config option

[mrjones2014]

Done!

[pwntester]

Awesome, thanks!