Migrate legacy keys folder on macOS

[keyz-tk]

Summary

See tkhq/go-sdk#10 and tkhq/go-sdk#11 for the SDK parts

Fixes #34

Test Plan

# Generate key via the old CLI
$ turnkey generate-api-key --organization e53d8453-aa58-4f2c-ad36-195f41fd756e
{
   "privateKeyFile": "/Users/keyan/Library/Application Support/turnkey/keys/default.private",
   "publicKey": "03328da93a7d6cc578321daeb87801315c1b59894149233f051e72c17c200a1298",
   "publicKeyFile": "/Users/keyan/Library/Application Support/turnkey/keys/default.public"
}

# Inspect old files
$ ls ~/Library/Application\ Support/turnkey/keys
default.meta    default.private default.public

# Run new CLI; migration script kicks off
$ ./build/turnkey gen
Legacy keys directory detected; will migrate keys to new location
- Legacy: /Users/keyan/Library/Application Support/turnkey/keys
- New: /Users/keyan/.config/turnkey/keys

Moved `default.meta` to /Users/keyan/.config/turnkey/keys/default.meta
Moved `default.private` to /Users/keyan/.config/turnkey/keys/default.private
Moved `default.public` to /Users/keyan/.config/turnkey/keys/default.public

Successfully migrated legacy keys directory.

{
   "error": "please supply an organization ID (UUID)"
}

# Legacy folder is now empty
$ ls ~/Library/Application\ Support/turnkey/keys
ls: /Users/keyan/Library/Application Support/turnkey/keys: No such file or directory

# New folder now contains `default.meta`, `default.private`, and `default.public`

# Running new CLI again: it no longer triggers the migration script
$ ./build/turnkey gen
{
   "error": "please supply an organization ID (UUID)"
}

# This one fails, because `default` already exists after the migration
$ ./build/turnkey gen --organization e53d8453-aa58-4f2c-ad36-195f41fd756e
{
   "error": "failed to store new API keypair: a keypair named \"default\" already exists; exiting"
}

$ ./build/turnkey gen --organization e53d8453-aa58-4f2c-ad36-195f41fd756e --key-name something-new
{
   "privateKeyFile": "/Users/keyan/.config/turnkey/keys/something-new.private",
   "publicKey": "03a1d88de7724b0d8d34a82aa6c681e2c42d315038d3ce80e8a788de9969bd6de0",
   "publicKeyFile": "/Users/keyan/.config/turnkey/keys/something-new.public"
}

[r-n-o]

Awesome!