Add Metatarz to the wallet directory

[metatarz]

Type of submission

• New wallet application
• Update to an existing wallet's feature support
• Third-party verification (confirmed, or disputed as unsupported) of an existing claim
• Marking a feature as not supported, with a reason

Summary

Add Metatarz to the Canton wallet directory

Checklist

• wallets/<wallet-name>.yaml added or updated (self-attestation)
• A heading added to proofs/<wallet-name>/<wallet-name>-self-attestation.md
  for every feature newly claimed as supported: true
• python scripts/generate_table.py was run and the regenerated
  WALLET_DIRECTORY.md is included in this PR (a CI check will catch
  this if it's missed, but running it locally saves a round trip)
• If applying for the first time: confirmed membership in
  #ext-temp-party-scaling and set slack_party_scaling_channel_joined: true
• If marking a feature as not supported: reason filled in under that
  feature's reason: field, not just left blank
• If this is a verification: the heading added to
  proofs/<wallet-name>/<wallet-name>-third-party-attestation.md
  reflects evidence gathered independently, not copied from the
  wallet's self-attestation, and result on the matching
  verified_by entry is set to verified or unsupported to match

Additional context