Provide callback for when parameter value is accessed

appcli.param(cast=...) only applies to values that are loaded from a config. It's not used for default values or values set directly in python. This definitely has its purpose—values from different config sources may need to be normalized differently. But it would also be good to have a way to treat all values in a consistent way, similar to @property.
I can think of two ways to do this. It might be good to support both, since they each have situations where they'd work well:
- decorator:
```
class Obj:

@appcli.param()
def a(self, a):
    assert is_valid(a)
    return a
```
- on_get/getter callback (goes with on_set/setter callback discussed in #4):
```
class Obj:

a = appcli.param(on_get=check_a)
```
This on_get callback is also unique from the cast callback in that it would provide the object itself as an argument.
It might also be good to allow a cast function to be specified specifically for values set directly in python. Here's one way to do it, although it feels a bit hacky.
```
class Obj:

  a = param(
          cast={
              DummyConfig: ...,
              'on_set': ...,
          },
  )
```
- Another way to do this would be to make parameters read-only unless __config__ includes a PythonConfig/SetAttrConfig/ApiConfig instance.
  - Some pseudo-code:
  - PythonConfig.load() would set a flag in get_meta(obj) indicating that values can be set.
  - Parameter setters would:
    - Complain if the above flag isn't set.
    - Otherwise set values with in a meta attribute (like they already do), or in the PythonConfig instance.
  - Pros:
  - Python is treated just like any other config; no longer a special case.
    - Can specify python cast-functions.
    - Can include python in picking
    - Although that kinda breaks how attributes usually work, and could be surprising.
    - Can put other configs above/below python.
  - Makes it possible for attributes to be read-only.
  - Cons:
  - Attributes are read-only by default.
  - Doesn't provide way to make only some attributes read-only. Parameter options are probably a better way to get implement a read-only feature.
  - Requirement to specify key for PythonConfig wouldn't make any sense, would need a way to avoid that.

kalekundert / byoc

Provide callback for when parameter value is accessed #1