feat(meta): add meta command for viewing/modifying key metadata

testdata/meta-decrypt.ct

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+# Decrypt an existing encrypted key
+$ pda set --encrypt hello@md world
+$ pda meta hello@md --decrypt
+$ pda meta hello@md
+  key: hello@md
+  secret: false
+  expires: never
+$ pda get hello@md
+world

testdata/meta-encrypt.ct

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+# Encrypt an existing plaintext key
+$ pda set hello@me world
+$ pda meta hello@me --encrypt
+$ pda meta hello@me
+  key: hello@me
+  secret: true
+  expires: never
+# Value should still be retrievable
+$ pda get hello@me
+world

testdata/meta-err.ct

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+# Error: key doesn't exist
+$ pda meta nonexistent@me --> FAIL
+FAIL cannot meta 'nonexistent@me': no such key
+
+# Error: --encrypt and --decrypt are mutually exclusive
+$ pda set hello@me world
+$ pda meta hello@me --encrypt --decrypt --> FAIL
+FAIL cannot meta 'hello@me': --encrypt and --decrypt are mutually exclusive
+
+# Error: already encrypted
+$ pda set --encrypt secret@me val
+$ pda meta secret@me --encrypt --> FAIL
+FAIL cannot meta 'secret@me': already encrypted
+
+# Error: not encrypted (can't decrypt)
+$ pda meta hello@me --decrypt --> FAIL
+FAIL cannot meta 'hello@me': not encrypted
+
+# Error: invalid TTL
+$ pda meta hello@me --ttl "abc" --> FAIL
+FAIL cannot meta 'hello@me': invalid ttl '"abc"': expected a duration (e.g. 30m, 2h) or 'never'

testdata/meta-ttl.ct

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+# Set TTL on a key, then view it (just verify no error, can't match dynamic time)
+$ pda set hello@mt world
+$ pda meta hello@mt --ttl 1h
+
+# Clear TTL with --ttl never
+$ pda set --ttl 1h expiring@mt val
+$ pda meta expiring@mt --ttl never
+$ pda meta expiring@mt
+  key: expiring@mt
+  secret: false
+  expires: never

testdata/meta.ct

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+# View metadata for a plaintext key
+$ pda set hello@m world
+$ pda meta hello@m
+  key: hello@m
+  secret: false
+  expires: never
+
+# View metadata for an encrypted key
+$ pda set --encrypt secret@m hunter2
+$ pda meta secret@m
+  key: secret@m
+  secret: true
+  expires: never