Add a link in the top of admin toolbar to MailHog on DDEV-Local

Admin Toolbar has a few meta-menus on the top bar, essentially leading to trays that contain the administration menu, shortcuts, or user account menu. Other modules add items here, like admin search or a Devel menu. I'd like to see a simple link to MailHog when we detect the site is running in DDEV (e.g. by looking for the DDEV_HOSTNAME environment variable).

For reference,

• Site URL: https://example.ddev.site/
• MailHog: https://example.ddev.site:8026/

DDEV-Local also supports phpMyAdmin, but I'm in the habit of using Sequel Pro and ddev sequelpro from the command line to launch that. I don't think we need to worry about a logo or anything ... just a simple link to open MailHog, preferably in a new tab.