`/createRoom` overly restricts appservices when creating an alias for the room

This issue has been migrated from [#15130](https://github.com/matrix-org/synapse/issues/15130).

---

### Description

See logs below - an appservice on my local machine tried to create a room with `room_alias_name` specified, but the request failed because "This application service has not reserved this kind of alias".

When the very same appservice calls `PUT /directory/room/%23the_alias:localhost` though, it is not prevented from doing so.

### Steps to reproduce

See description.

### Homeserver

localhost

### Synapse Version

matrixdotorg/synapse:v1.76.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

postgresql, in a docker container

### Workers

Single process

### Platform

N/A

### Configuration

N/A

### Relevant log output

```shell
element-dev-stack-eledev-synapse-1 | 2023-02-21 22:12:23,673 - synapse.http.server - 108 - INFO - POST-22 - SynapseError: 400 - This application service has not reserved this kind of alias.
element-dev-stack-eledev-synapse-1 | 2023-02-21 22:12:23,673 - synapse.access.http.8008 - 460 - INFO - POST-22 - 172.26.0.5 - 8008 - {@example_user_1676066190192:localhost} Processed request: 0.026sec/0.000sec (0.001sec, 0.001sec) (0.001sec/0.018sec/4) 97B 400 "POST /_matrix/client/v3/createRoom?user_id=@example_user_1676066190192:localhost HTTP/1.0" "okhttp/4.10.0" [0 dbevts]
```

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

The spec does not enforce this restriction anywhere: the `/createRoom` request should not be failing.

Start by comparing authorization for POST /_matrix/client/v3/createRoom with room_alias_name against PUT /directory/room/%23the_alias:localhost, using the reported appservice behavior as the reproduction. The work is done when the create-room request no longer rejects an unreserved alias where the specification permits it, while the existing directory behavior remains consistent.

Written by the indexing model from the issue text.