fix(email)!: attachments

[ioanniskemerlis]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other (please describe)

Does this PR introduce a breaking change?

• Yes
• No

• changed the type of attachments from:
  attachments?: string[];

to

attachments?: {
  filename: string;
  contentType?: string;
  content?: Buffer;
  href?: string;
  httpHeaders?: Record<string, string>;
}[];

• ISendEmailParams went from attachments?: string[] to Attachment[].

• SendEmailParams.attachments in email.proto changed from repeated string to repeated Attachment

any caller will now fail type checking and may also fail at runtime if not updated.
callers who never set attachments are largely unaffected (still optional).

The PR fulfills these requirements:

• It's submitted to the main branch
• When resolving a specific issue, it's referenced in the PR's description (e.g. fix #xxx, where "xxx" is the issue number)

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature

As I understand it (not my branch), we replaced an unspecified string[] with a proper attachment message (from node mailer) so we can send binary content, correct MIME types, and URL-based attachments.
The only thing you could send through gRPC was text strings and then the email module did try to get the file from storage / a path the email service could read. Now we can send payload over gRPC as bytes in a proper attachment shape.

[kkopanidis]

If I were to merge this with this title, it'd trigger a larger bump than the intended. Considering this functionality is already broken, I'll change the title slightly