feat: Add arbitrary header overrides for attachments

[jarrodmoldrich]

This allows bamboo adapters to be accept overriding headers for special use cases. Updates to those adapters would be required to use this functionality.

For e.g., in the specific use-case of adding inlined attachments, some special case headers are required. Now it's possible to add X-Attachment-Id and override the Content-Disposition header with inline to permit the proper display of inlined content, without an email client showing them as explicit attachments for the user to download.

Closes #502 #430

Related PRs: https://github.com/kalys/bamboo_ses/pull/43 https://github.com/DockYard/elixir-mail/pull/137

[doomspork]

Hi @jarrodmoldrich! Now that BEAM Community is taking over Bamboo we're working through these older PRs. Would you like to update this PR or should we close?

[jarrodmoldrich]

@doomspork Hi there! I probably won't be testing this against SES as I've retired the infra I was using, but I'll get the PR up-to-date and to a working state in the coming weeks.

[doomspork]

Amazing, thank you @jarrodmoldrich!

[jarrodmoldrich]

@doomspork This is ready for another look (assuming it passes your CI)

[github-actions[bot]]

This pull request has been automatically marked as "stale:discard". If this pull request is still relevant, please leave any comment (for example, "bump"), and we'll keep it open. We are sorry that we haven't been able to prioritize reviewing it yet. Your contribution is very much appreciated!.

[jarrodmoldrich]

@doomspork Has this work been deprecated? I did update the PR but received no feedback on it.