[feature] allow modifying jitter at a per-watch level

### **Version and OS**
0.45.22/docker

### **Is your feature request related to a problem? Please describe.**
Recently I created a watch to get updates for something very time sensitive.
Since i overrode the global time between checks to 5 minutes, I expected the watch to alert me of changes quickly.

It wasn't until i manually checked the target website when i got to know the page was updated with new information.

I was wondering why didn't changedetection alert me earlier. I noticed that the "last checked time" shown in the dashboard was 36 minutes ago when it should have been less than 5 minutes (because time b/w check was set to 5 minutes)

I eventually figured out it was because in the settings page i kept the jitter value to 1 hour. I did not remember i even set this value.

My default global settings are very relaxed to lower chances of my ip getting rate limited. 12 hours for checks and then a 1 hour jitter. Obviously i override the time between checks when needed for a specific site but i forgot about the existence of a jitter value until this happened

So now if i have to create a time sensitive check i would also have to adjust the global jitter.

### **Describe the solution you'd like**
Just like how we have an option to override time between checks at a per-watch level, have an option to configure the jitter as well.

### **Describe the use-case and give concrete real-world examples**
The site i was monitoring was booking status of a friend's train tickets a few hours before departure. (dont worry they decided to cancel the tickets instead since it wasn't confirmed. 😺 )

### **Additional context**
NA

No files or tests are named. Start by tracing the existing per-watch override for time between checks and the global jitter setting; done means a watch can configure jitter independently and a time-sensitive watch is checked according to that per-watch value.

Written by the indexing model from the issue text.