[async-mqtt] Update to 1.0.3.

[redboltz]

If this PR updates an existing port, please uncomment and fill out this checklist:

• Changes comply with the maintainer guide
• SHA512s are updated for each updated download
• The "supports" clause reflects platforms that may be fixed by this new version
• Any fixed CI baseline entries are removed from that file.
• Any patches that are no longer applied are deleted from the port's directory.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
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END OF PORT UPDATE CHECKLIST (delete this line) -->

This PR adds version 1.0.3 for exisitng async-mqtt port.