Per-recipient metadata

[medmunds]

PR #134 proposes a new merge_metadata feature, which allows providing unique metadata for each recipient in a batch send. The PR implements merge_metadata for SendGrid.

Should implement per-recipient metadata for these ESPs that can support it:

• Mailgun (somewhat complicated: Anymail uses "custom variables" for metadata; these can be set per-recipient using "recipient variables"; need to avoid conflicts with merge_data, which also uses recipient variables)
• Mailjet (batch send is an array of individual messages, including Mj-EventPayLoad)
• Mandrill (top-level recipient_metadata is exactly the same form as proposed merge_metadata)
• Postmark (batch send is an array of individual messages, including metadata)
• SendGrid (individual custom_args in personalizations array are merged with top-level custom_args: PR Support for per-message meta data for SendGrid #134)
• SparkPost (individual metadata in recipients array are merged with top-level metadata)

It won't be possible to implement merge_metadata for these ESPs:

• Amazon SES: not possible (Anymail uses X-Metadata custom header; SendBulkTemplatedEmail doesn't currently support custom headers)
• SendinBlue: not applicable (SendinBlue's transactional APIs don't support batch sending)

[medmunds]

Released in Anymail v6.0.