Improve format of `synapse_review_recent_signups` output to easier parsing as JSON or XML

This issue has been migrated from [#10434](https://github.com/matrix-org/synapse/issues/10434).

---

**Description:**
The recently added `synapse_review_recent_signups` is great, but my homeserver I have use bridges, so they constantly creates a lot of virtual accounts such as `@_telegram_123456:example.com` and `@_slackpuppet___t02_f_l_v1_a_n=2d_u027_h616_b41:example.com`, that shown as long list in output of `synapse_review_recent_signups` script, and I can't find the real registered users between that accounts.

And the output format is not so easy to parse, for example, via `grep`, `jq`, or any other cli tool:
```
# Created: 2021-07-13 18:55:08
# Email: None
# IPs:
# Number joined public rooms: 0
# Number joined private rooms: 1
#
@_slackpuppet___t02_f_l_v1_h_n=2d_u028_a9_e2_k_p_b:example.com

# Created: 2021-07-13 19:47:54
# Email: None
# IPs:
# Number joined public rooms: 0
# Number joined private rooms: 0
#
@_slackpuppet___t02_f_l_v1_h_n=2d_u027_h616_b47:example.com

# Created: 2021-07-13 20:17:02
# Email: None
# IPs:
# Number joined public rooms: 0
# Number joined private rooms: 1
#
@_telegram_1234567:example.com
```

To solve this problem will be good to add some parseable formatters to output, for example - JSON or XML format.

Alternative can be adding support for filtering account name by regex into the script, but this will add more complexity to it and anyway don't give all flexibility like custom output parsers.