Make a `DEFAULT_TIMEOUT` externally modifiable

[coretl]

This would be useful in both tests, and as a general "something timed out, just try increasing the timeout a bit and see if it helps".
Need to keep a global reference that can temporarily (via context manager) or permanently (with function call) be set to a new value

Suggestion for implementation:

# core/_util.py
class OverridableTimeout(SupportsFloat):
    def __init__(self, value: float):
        self.value = value
    def set_to(self, value: float) -> ContextManager:
        original_value = self.value
        self.value = value
        @functools.context_manager
        def revert():
            yield
            self.value = original_value
        return revert
    def __float__(self) -> float:
        return self.value

DEFAULT_TIMEOUT = OverridableTimeout(10.0)
         
# test_my_device.py
from ophyd_async.core import DEFAULT_TIMEOUT

with DEFAULT_TIMEOUT.set_to(0.01):
    with pytest.raises(TimeoutError):
        # Somethere this calls a utility function with timeout=DEFAULT_TIMEOUT
        await device.read()

# at runtime
>>> RE(bps.rd(device))
# uh oh it failed
>>> from ophyd_async.core import DEFAULT_TIMEOUT
>>> DEFAULT_TIMEOUT.set_to(20.0)
>>> RE(bps.rd(device))
# yay it worked

Acceptance Criteria

• Code and tests written
• Docs page written to show usage in both tests and at runtime