Feature suggestion: defining arbitrary functions only using gin

This may be kinda hacky, but what if I wanted to use some function and only define it via gin file,
so this is the solution I came up with:
Function:
```utils.py
#utils.py
@gin.configurable
def arbitrary_func(module_name, func_name, kwargs):
"""Returns a function from a module."""
module = importlib.import_module(module_name)
func = getattr(module, func_name)
return func(**kwargs)
```
Gin file:
```lr_schedule.gin
import utils
lr_schedule/utils.arbitrary_func:
module_name = "optax"
func_name = "linear_schedule"
kwargs = { "init_value":0.005, "end_value":0.1, "transition_steps": 10000 }

optimizer_config.AdamConfig:
lr_schedule = @lr_schedule/utils.arbitrary_func()
```
let me know what you think.

Start by reviewing the proposed utils.py helper and lr_schedule.gin example, then trace gin's configurable and reference-resolution entry points and related tests. Define the supported arbitrary-function behavior and its configuration syntax before determining what tests should demonstrate that the feature is complete.

Written by the indexing model from the issue text.