bugfix: incorrect raw byte conversion to string

[IdrisHanafi]

Summary & Motivation (Problem vs. Solution)

This PR fixes an issue where decrypted private keys were incorrectly converted to UTF-8 strings instead of the expected hexadecimal format...The current implementation works for regular text like a mnemonic.

This change introduces a new flag in the decrypt command called --hex-output which ensures that raw bytes are correctly converted as hexadecimal string representations.

Test

(Don't worry below is a throwaway ETH account purely for testing)
Before:

$ turnkey decrypt --export-bundle-input test.json --organization $ORG_ID --signer-quorum-key $SIGNER_QUORUM_KEY
"\ufffd[\ufffd.\ufffd\ufffd|\ufffd\u0005\ufffd\ufffd\ufffda\u003eό\ufffd5y9....<Redacted>"

After:

$ go run main.go decrypt --export-bundle-input ~/dev/turnkey-test/test.json --organization $ORG_ID --signer-quorum-key $SIGNER_QUORUM_KEY --hex-output
"fc5b812e96...<Redacted>"