Customizable breakpoint file location

First, love to use pudb daily, thanks :heart:

## Issue
Currently, breakpoint is saved globally, usually in `~/.config/pudb/saved-breakpoints-3.X`.

But in most use cases, breakpoints are natual to be set by project.
- Breakpoint set for project A means nothing for project B, and vice versa

So while debugging for several projects, the breakpoint file is getting bigger and bigger. This is not scalable nor ideal.

## Suggest
It might be wonderful if there's way to set breakpoint by session. There are plenty of options I can imagine.

### pudb cli option to get breakpoint file from user
```
pudb --breakpoint ./my_breakpoint.txt ...
```
I see #539 suggested to do this for configuration. This suggestion is breakpoint version of #539.
I think configuration is mostly global, while breakpoint is natively per project. So getting local breakpoint file from user seems natual.
### Convention for project-wise breakpoint file location to override global setting
Like `.pudb_saved_breakpoint` in project root overrides global breakpoint file, if exists.
### pudb cli option to take breakpoint at runtime (#500)
#500 might be one option (but I prefer getting file as argument)

Start by tracing pudb's command-line handling and breakpoint persistence; the issue does not name specific files or tests. First resolve whether the supported behavior should be a CLI path, a project-root convention, or runtime input, then verify that project or session breakpoints can be isolated from the global file.

Written by the indexing model from the issue text.