Add new syscall os dashboard mailbox

[yrichard-ledger]

Description

Define a new syscall between UX and dashboard.
No impact on applications.

Changes include

• Bugfix (non-breaking change that solves an issue)
• [ X ] New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)
• Tests
• Documentation
• Other (for changes that might not fit in any category)

Breaking changes

Please complete this section if any breaking changes have been made, otherwise delete it.

Additional comments

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 35.76%. Comparing base (0707349) to head (cf75494).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #773   +/-   ##
=======================================
  Coverage   35.76%   35.76%           
=======================================
  Files          13       13           
  Lines        1591     1591           
=======================================
  Hits          569      569           
  Misses       1022     1022           

Flag	Coverage Δ
unittests	35.76% <ø> (ø)

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