Structure Outlook conversion API around source metadata

[bbottema]

Context

Issue #609 surfaced a valid archival / extraction use case: callers converting Outlook .msg files sometimes need access to source headers and Outlook-specific metadata that Simple Java Mail intentionally does not copy into Email#getHeaders().

Scope

Clean up the Outlook converter API so this data is available through an explicit secondary conversion result instead of making the ignored-header list configurable.

• Add a public result type for Outlook conversions.
• Expose source .msg headers and selected Outlook metadata for archive / indexing use cases.
• Keep converted Email headers focused on headers that are safe and meaningful for the normalized Simple Java Mail model.
• Preserve compatibility for the existing conversion methods.

Fixes #609

[bbottema]

Implemented by adding an explicit Outlook conversion result API instead of making the ignored-header list configurable.

Summary:

• Added OutlookEmailConversionResult as the public result type for Outlook .msg conversions that need source metadata.
• Added OutlookMessageData for source .msg headers, raw header text, display recipient fields, Outlook dates, and MAPI property metadata.
• Added EmailConverter.outlookMsgToEmailBuilderWithOutlookData(...) overloads for String, File, and InputStream.
• Kept converted Email#getHeaders() focused on safe normalized headers, so structural source headers such as To, Cc, Date, and transport headers are available for inspection without being replayed into outgoing email.

Usage example:

import org.simplejavamail.api.email.Email;
import org.simplejavamail.api.outlook.OutlookEmailConversionResult;
import org.simplejavamail.api.outlook.OutlookMessageData;
import org.simplejavamail.converter.EmailConverter;

import java.io.File;
import java.util.List;

OutlookEmailConversionResult result =
        EmailConverter.outlookMsgToEmailBuilderWithOutlookData(new File("message.msg"));

Email email = result.buildEmail();
OutlookMessageData outlookData = result.getOutlookMessageData();

List<String> receivedHeaders = outlookData.getHeaderValues("Received");
List<String> dateHeaders = outlookData.getHeaderValues("Date");
String rawHeaders = outlookData.getRawHeaders();

Use Email for the normalized mail model, body, recipients, and attachments. Use OutlookMessageData for archival/indexing use cases that need the original Outlook source metadata.

[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