[lldb-dap][NFC] Add Breakpoint struct to share common logic. (#80753)

This adds a layer between `SounceBreakpoint`/`FunctionBreakpoint` and
`BreakpointBase` to have better separation and encapsulation so we are
not directly operating on `SBBreakpoint`.

I basically moved the `SBBreakpoint` and the methods that requires it
from `BreakpointBase` to `Breakpoint`. This allows adding support for
data watchpoint easier by sharing the logic inside `BreakpointBase`.