Move JsonDict import to type-checking path only

I ran into the usual case where my `synapse_rust` module was outdated and
needed to rebuild them before running the Complement tests.

However, instead of getting the typical error of `Rust module outdated.
Please rebuild using `poetry install`, I instead got the following:

```
COMPLEMENT_DIR="../complement" ./scripts-dev/complement.sh -run
  "TestMSC4429ProfileUpdates"
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/home/work/code/synapse/synapse/util/__init__.py", line 41, in <module>
      from synapse.types import JsonDict
    File "/home/work/code/synapse/synapse/types/__init__.py", line 70, in <module>
      from synapse.synapse_rust.types import Requester
  ModuleNotFoundError: No module named 'synapse.synapse_rust.types'; 'synapse.synapse_rust' is not a package.
```

which is much more confusing for a irregular contributor to
diagnose than the one that tells you to run `poetry install`.

The cause was an edge case: I did not yet have the
`synapse.synapse_rust.types` module, which was added only recently in
https://github.com/element-hq/synapse/pull/19828. So you could argue
this is a one-time hurdle.

However, the need to import `synapse.synapse_rust.types` solely comes
from needing to import `JsonDict`, which is only used for typechecking!

Hence moving the import under `if typing.TYPE_CHECKING`. With this, I
would have received the more noob-friendly error message instead of one
about a `synapse_rust.types` module not existing.

synapse/util/__init__.py

@@ -38,10 +38,8 @@ from matrix_common.versionstring import get_distribution_version_string
 from twisted.internet import defer
 from twisted.python.failure import Failure
 
-from synapse.types import JsonDict
-
 if typing.TYPE_CHECKING:
-    pass
+    from synapse.types import JsonDict
 
 logger = logging.getLogger(__name__)
 
@@ -189,7 +187,7 @@ def split_dict_to_fit_to_size(
     *,
     soft_max_size: int,
     wrapping_object_size: int = 2,
-) -> Iterator[tuple[dict[str, JsonDict], int]]:
+) -> Iterator[tuple[dict[str, "JsonDict"], int]]:
     """Splits a dict up into a list of dicts, each of which is small enough to
     fit into the given size when encoded as JSON. Every entry in the original
     dict is in exactly one of the resulting dicts.