New feature: per-recipient merge_headers

[medmunds]

Several ESPs' batch send APIs allow specifying different email headers for individual recipients. Anymail should add a new merge_headers message option providing normalized support for this feature:

message = AnymailMessage(
    to=["[email protected]", "Bob <[email protected]>"],
    ...,
    # Common headers for all recipients:
    headers={
        "List-Unsubscribe-Post": "List-Unsubscribe=One-Click",
    },
    # Recipient-specific headers:
    merge_headers={
        "[email protected]": {
            "List-Unsubscribe": "<https://example.com/unsubscribe/98765abc>",
        },
        "[email protected]": {
            "List-Unsubscribe": "<https://example.com/unsubscribe/12345xyz>", 
        },
    },
)

As the example shows, this is particularly useful for List-Unsubscribe headers that include a token identifying the recipient. The new merge_headers property is meant to parallel Anymail's existing merge_metadata and merge_data.

Tasks:

• [x] Base implementation
• [x] Amazon SES (only works with template)
  • [ ] Add integration test
• [ ] Brevo
• [ ] MailerSend ??? (can MailerSend "personalizations" substitutions be used in headers?)
• [ ] Mailgun ??? (do Mailgun "replacement values" work in headers?)
• ~~Mandrill~~ (batch API doesn't seem to support per-recipient headers)
• ~~Postal~~ (no batch send API)
• [ ] Mailjet
• [ ] Postmark
• [ ] Resend
• [ ] Sendgrid
• ~~Sparkpost~~ (batch API doesn't seem to support per-recipient headers)
• [ ] Unisender Go ("substitutions" is documented for List-Unsubscribe header only)
• [ ] Documentation
  • [ ] Add note to "Limitations" section for each unsupported ESP

In addition to providing the idea, @carrerasrodrigo has already implemented the base and Amazon SES support in #371. (Thanks!)