email: conditional recipients in rendered email metadata

[juleswg23]

We have recently added support for multiple emails rendered from a single document. This next step will allow any/all emails to have conditional recipients.

I am flexible on the format of accepted quarto input, here is a draft of how it might look:

---
title: Email Test Document
author: Charles Teague
format: email
---

The report content. Anything that is here is not part of the email message.

::: {.email}

::: {.subject}
The subject line.
:::

```{python}
import datetime

# Get today's date
today = datetime.date.today()

if today.weekday() < 5:
    recipients = ["test1@test.com", "test2@test.com", "test3@test.com"]
else:
    recipients = ["test4@test.com", "test5@test.com", "test6@test.com"]
```

:::{.recipients}
`{python} recipients`
:::

::: {.email-text}
An optional text-only version of the email message..
:::

The HTML email content. Here you can add code cells, produce images, and write accompanying text.

:::

Any additional report content not part of the email message.

Multiple emails here: #13882