Provide the ability to send pre-encoded attachments

[dmittriy13]

Issue

We store attachments encoded in base64 in the database; the current library API does not allow sending pre-encoded attachments. Instead, we have to decode them and then provide them to the library to re-encode just before sending them.

Steps to Reproduce

1. Create an builder using EmailBuilder.startingBlank().
2. Add an attachment that is already encoded, for example in base64
3. Call EmailBuilder#build for create new email.
4. Call Mailer#send for send email.

Expected Behavior

The attachment will not be re-encoded, but will be sent in the current encoding.

Actual Behavior

The attachment will be re-encoded

Request

It is necessary to be able to indicate that the attachment is already encoded and specify the encoding. The library should use jakarta.mail.internet.PreencodedMimeBodyPart as a container for such attachments.

[bbottema]

Implemented on feature/recipient-builder in commit 1269966e.

Summary:

• Added explicit withPreEncodedAttachment(...) builder APIs for byte arrays and DataSource payloads.
• Added matching withPreEncodedEmbeddedImage(...) APIs, because inline images are MIME body parts too and can hit the same double-encoding problem.
• Kept the existing withAttachment(..., ContentTransferEncoding) behavior separate: that remains the raw-data path where Simple Java Mail controls the outgoing transfer encoding.
• When a resource is marked pre-encoded, Simple Java Mail now renders it through PreencodedMimeBodyPart so the stored payload is preserved instead of encoded again.

Usage:

String encodedPdf = "..."; // already base64 encoded

Email email = EmailBuilder.startingBlank()
    .from("sender@example.com")
    .to("receiver@example.com")
    .withPlainText("See attachment")
    .withPreEncodedAttachment(
        "report.pdf",
        encodedPdf.getBytes(StandardCharsets.US_ASCII),
        "application/pdf",
        ContentTransferEncoding.BASE_64)
    .buildEmail();

Inline image variant:

Email email = EmailBuilder.startingBlank()
    .from("sender@example.com")
    .to("receiver@example.com")
    .withHTMLText("<img src='cid:logo'>")
    .withPreEncodedEmbeddedImage(
        "logo",
        encodedLogo.getBytes(StandardCharsets.US_ASCII),
        "image/png",
        ContentTransferEncoding.BASE_64)
    .buildEmail();

[bbottema]

Released in Simple Java Mail v9.0.0, which is now available from Maven Central:

https://repo1.maven.org/maven2/org/simplejavamail/simple-java-mail/9.0.0/

Release notes:
https://github.com/bbottema/simple-java-mail/releases/tag/9.0.0