feat: add progress indicator for encrypting messages

[schorschii]

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Contribution Summary

Linked Issue/Ticket: #5763
RFC / Technical Design (if applicable):

Description

When sending images (> 5 MB) with enabled PGP encryption, it takes a while until the mail is sent, but no progress is reported to the user. This is misleading, as nothing happens on the screen until the email has been sent. This change introduces a simple progress indicator while encryption is running.

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Contribution Checklist

• I have read and affirm that my contribution adheres to Mozilla’s Community Participation Guidelines
• This contribution is in Kotlin where possible
• This contribution does not use merge commits
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• This contribution includes tests for any new functionality, and maintains tests for any updated functionality.
• This contribution adheres to our Engineering process (RFC/Technical Design/ADR)
• This PR has a descriptive title and body that accurately outlines all changes made, and contains a reference to any issues that it fixes (e.g. Closes #XXX or Fixes #XXX).

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