Add batch sending support for Resend

[medmunds]

Resend recently announced a new batch sending API. Anymail should add support for it.

Resend's batch send API basically takes an array the same payload structure used for the regular send API. So it:

• Wouldn't support Anymail's merge_data (because Resend doesn't support ESP-rendered templates even for regular send)
• Would support Anymail's merge_metadata (batch send supports custom headers, which Anymail uses for metadata and tags with Resend)
• Doesn't support attachments "yet" (need to test if attachments field causes error, or is just silently ignored right now)

[medmunds]

(Released in Anymail 10.3)