PYTHON-4834 Add __repr__ to pymongo.operations.IndexModel

[aclark4life]

Via @timgraham

While checking --debug-sql output, I saw: db.schema_authorcharfieldwithindex.create_indexes([<pymongo.operations.IndexModel object at 0x7fa7285d0370>]) . I thought you might like to add a proper pymongo.operations.IndexModel.__repr__()  to improve this output. I'd like to see something like https://github.com/django/django/blob/4d11402932eca570850bdfa58a71eb59fc62275a/django/db/models/indexes.py#L193-L207.
indexes.py
    def __repr__(self):
        return "<%s:%s%s%s%s%s%s%s>" % (
            self.__class__.__qualname__,
            "" if not self.fields else " fields=%s" % repr(self.fields),
            "" if not self.expressions else " expressions=%s" % repr(self.expressions),
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<https://github.com/django/django|django/django>django/django | Added by GitHub

https://jira.mongodb.org/browse/PYTHON-4834

[timgraham]

There are no changes required in this project, so we don't need to track it here.

[aclark4life]

@timgraham In that case, please add any feedback you may have about it to mongodb/mongo-python-driver#1909 I've requested a review from @Jibola over there.