please expose slackApiUrl

[glimchb]

as mentioned here https://tools.slack.dev/node-slack-sdk/web-api/#custom-api-url
one can customize slack API URL like options.slackApiUrl = 'http://localhost:8888/api/';
this is not exposed in the action yet

addition in this area https://github.com/slackapi/slack-github-action/blob/main/src/client.js#L27

please do

[zimeg]

Hey @glimchb 👋

Thanks for this feature request - I think it'd be useful to have! We'll look into adding this to the next release and will share updates here 🚀