improve(logger): ignore invalid slack webhook URLs

[bmzig]

Motivation

An end user may want to instantiate an UMA logger with a slack transport but only send slack messages for specific notification paths. This is not currently possible since we will unconditionally post a text payload to either the config's escalation path webhook URL or the default webhook URL.

Summary

This PR now allows invalid URLs to be defined in SLACK_CONFIG, and, if an invalid URL is selected as the webhookUrl, log will exit early.