feat(cli,core): trace the keys that open My Files in pdfs diagnose

When the session is stored and unlocked but the daemon does not answer,
pdfs diagnose asks Proton for a key report and prints a Keys section:
each account key, each address key and the account key its token is
for, and the address key the My Files share is encrypted to. It holds
key IDs only, so it can be posted in an issue.

Refs #40

docs/CLI.md

@@ -171,7 +171,7 @@ commands are for scripts and for cases where the mount is not available.
 
 | Command | Description |
 |---|---|
-| `pdfs diagnose` | Check the installation and print a report. Works without the service. Exits non-zero if a check fails. |
+| `pdfs diagnose` | Check the installation and print a report. Works without the service. When the service does not answer, also asks Proton which of your keys open. Exits non-zero if a check fails. |
 | `pdfs diagnostics` | What a running service is doing now: worker threads, queues, requests in flight, memory. For a service that has stopped answering. |
 | `pdfs cache inspect [--deep]` | Database size, reclaimable space, row counts, cache use against the limit. `--deep` also runs SQLite's integrity check (slow). |
 | `pdfs cache vacuum` | Compact the database. Needs free space for a second copy of it while it runs. |