vapix-sh: Add example that uses shellscript only and not C

[joakimr-axis]

Describe your changes

This example does the same thing as the VAPIX example written in C, but uses a POSIX shellscript to implement the application instead. It shows how simpler applications can be written with just a shellscript but still bring value, which is good for those who are not C-savvy. (Whereas those who want to build more advanced applications with VAPIX would probably opt for the C version to build upon.)

Checklist before requesting a review

• [x] I have performed a self-review of my own code
• [x] I have verified that the code builds perfectly fine on my local system
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have verified that my code follows the style already available in the repository
• [x] I have made corresponding changes to the documentation