Allow `show techsupport` to save with a custom filename

[donggyu-nexthop]

Title. As of today, all techsupport dumps are saved under /var/dump/<DEVICE_HOST_NAME>_YYYYMMDD_HHMMSS.tar.gz. However, this is not very human-friendly (especially if there are multiple dumps with similar timestamps).

The request is to implement an optional argument that allows users to specify the filename under /var/dump at the time of invoking show techsupport.

The benefits I see are:

• scripts can generate and ues deterministic filenames without parsing the directory contents of /var/dump and matching timestamps
• more human readable, especially if multiple dumps are saved in the dump directory