`/createRoom`: "UnicodeEncodeError" and Internal server error when having specific garbage in `id_access_token` of a 3pid invite

This issue has been migrated from [#13512](https://github.com/matrix-org/synapse/issues/13512).

---

### Description

When sending some unicode characters in a 3pid invite on `/_matrix/client/v3/createRoom` the server returns an "Internal server error".

Such an invalid invite can be: `{"invite_3pid":[{"address":"localhost","id_access_token":"c��","id_server":"","medium":""}],"name":""}`

Confusing enough, the room actually will be created. However, no invite is issued.

### Steps to reproduce

- Send an invalid 3pid invite (see above for an example) to the createRoom endpoint.

### Homeserver

localhost

### Synapse Version

{"server_version":"1.64.0","python_version":"3.9.13"}

### Installation Method

Docker (matrixdotorg/synapse)

### Platform

Kubernetes with postgres and the official docker image

### Relevant log output

```shell
2022-08-11 22:41:50,084 - synapse.http.client - 455 - INFO - POST-8133- Error sending request to GET https:///_matrix/identity/v2/hash_details?access_token=: ValueError invalid hostname:
2022-08-11 22:41:50,084 - synapse.handlers.identity - 632 - WARNING - POST-8133- Error when looking up hashing details: invalid hostname:
2022-08-11 22:41:50,088 - synapse.http.server - 183 - ERROR - POST-8133- Failed handle request via 'RoomCreateRestServlet':
Traceback (most recent call last):
File "/usr/local/lib/python3.9/site-packages/twisted/internet/defer.py", line 1660, in _inlineCallbacks
result = current_context.run(gen.send, result)
StopIteration: {('m.room.create', ''): '$5z43xevRXglYCpB0aEaT8kl_4jLMENhOIGHQGk4E3wE', ('m.room.join_rules', ''): '$4d4a3uZvsS-_7N9QPQjMV7to3svMroxwHDhFBZD-iww', ('m.room.name', ''): '$3KU_XxJUR0mQmOMe64WbSOcohDbBYo0rb7SR7gwKoJA', ('m.room.member', '@fuzzer:localhost'): '$h9EIRCWvHHGChajNL_BLQ5NIo27b6drGsU-YPocIQxU'}

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/local/lib/python3.9/site-packages/synapse/http/server.py", line 366, in _async_render_wrapper
callback_return = await self._async_render(request)
File "/usr/local/lib/python3.9/site-packages/synapse/http/server.py", line 572, in _async_render
callback_return = await raw_callback_return
File "/usr/local/lib/python3.9/site-packages/synapse/rest/client/room.py", line 91, in on_POST
info, _ = await self._room_creation_handler.create_room(
File "/usr/local/lib/python3.9/site-packages/synapse/handlers/room.py", line 990, in create_room
) = await self.hs.get_room_member_handler().do_3pid_invite(
File "/usr/local/lib/python3.9/site-packages/synapse/handlers/room_member.py", line 1469, in do_3pid_invite
event, stream_id = await self._make_and_store_3pid_invite(
File "/usr/local/lib/python3.9/site-packages/synapse/handlers/room_member.py", line 1553, in _make_and_store_3pid_invite
) = await self.identity_handler.ask_id_server_for_third_party_invite(
File "/usr/local/lib/python3.9/site-packages/synapse/handlers/identity.py", line 865, in ask_id_server_for_third_party_invite
{"Authorization": create_id_access_token_header(id_access_token)},
File "/usr/local/lib/python3.9/site-packages/synapse/handlers/identity.py", line 944, in create_id_access_token_header
bearer_token.encode("ascii")
UnicodeEncodeError: 'ascii' codec can't encode characters in position 8-9: ordinal not in range(128)
```

### Anything else that would be useful to know?

_No response_