Add TaskOnKart load type

[kitagry]

Add load type for TaskOnKart.

class SampleTask(gokart.TaskOnKart):
    str_task: gokart.TaskOnKart[str] = gokart.TaskInstanceParameter()
    int_task: gokart.TaskOnKart[int] = gokart.TaskInstanceParameter()

    def requires(self):
        return dict(str=self.str_task, int=self.int_task)

    def run(self):
        s = self.load(self.str_task)  # This type is inferred with "str"
        i = self.load(self.int_task)  # This type is inferred with "int"

SampleTask(
    str_task=StrTask(),  # mypy ok
    int_task=StrTask(),  # mypy error: Argument "int_task" to "StrTask" has incompatible type "StrTask"; expected "TaskOnKart[int]
)

TODO

• [x] should check TaskOnKart which is arguments of load method is in requires results
• [x] write docs

[mski-iksm]

@kitagry Thank you for suggesting great feature!

How about adding new document about this feature as in https://github.com/m3dev/gokart/pull/391#issue-2497559110 ?