Internal 3PID requests as urlencoded causing problems with mxisd

*This issue was originally created by [**@marcialvieira**](https://github.com/marcialvieira) at .*

### Background information

- **Dendrite version or git SHA**: 0.5.0
- **Monolith or Polylith?**: Monolith
- **SQLite3 or Postgres?**: Postgres
- **Running in Docker?**: Docker (matrixdotorg/dendrite-monolith:latest)
- **`go version`**: go1.17

### Description

Attempting to associate an email address to an account via 3PID, the request to the identity server (mxisd) is sent as "form-urlencoded" instead of "application/json" causing a failure.

```
[XNIO-1 task-16] INFO io.kamax.mxisd.http.undertow.handler.BasicHttpHandler - Request POST http://id.homeserver/_matrix/identity/api/v1/validate/email/requestToken - Error M_INVALID_JSON: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 1 path $
```

Captured network data is `client_secret=MYlT2A5eS5l1xxaU8QUIi1skJpx3KT9J&email=xxx%40email.com&send_attempt=1`

https://github.com/matrix-org/dendrite/blob/master/clientapi/threepid/threepid.go#L62,L74

and it is documented as "application/json".

https://matrix.org/docs/spec/identity_service/r0.2.0#post-matrix-identity-api-v1-validate-email-requesttoken

### Steps to reproduce

```
curl 'http://127.0.0.1:8008/_matrix/client/r0/account/3pid/email/requestToken' -i -H 'content-type: application/json' -H 'Cookie: ph_phc_Jzsm6DTm6V2705zeU5dcNvQDlonOR68XvX2sh1sEOHO_posthog=%7B%22distinct_id%22%3A%2217c1429fd3183d-0b60d59a2fc38e-192b1d0f-1ea7ee-17c1429fd3298a%22%2C%22%24device_id%22%3A%2217c1429fd2d6f6-0639b2633da76-192b1d0f-1ea7ee-17c1429fd2e937%22%2C%22appVersion%22%3A%22v1.8.5%22%2C%22appPlatform%22%3A%22Web%20Platform%22%2C%22%24session_recording_enabled%22%3Afalse%2C%22%24active_feature_flags%22%3A%5B%5D%7D' --data-raw '{"email":"xxx@email.com","client_secret":"MYlT2A5eS5l1xxaU8QUIi1skJpx3KT9J","send_attempt":1,"id_server":"identity.homeserver"}'
HTTP/1.1 500 Internal Server Error
Access-Control-Allow-Headers: Origin, X-Requested-With, Content-Type, Accept, Authorization
Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS
Access-Control-Allow-Origin: *
Content-Type: application/json
Date: Fri, 08 Oct 2021 23:56:30 GMT
Content-Length: 55

{"errcode":"M_UNKNOWN","error":"Internal Server Error"}
```

Start in clientapi/threepid/threepid.go around lines 62-74 and compare the outgoing request with the Matrix identity-service specification linked in the issue. Reproduce the failure using the supplied curl command and verify that the mxisd request is sent as JSON and no longer produces the reported M_INVALID_JSON or 500 response.

Written by the indexing model from the issue text.