[transfer-hook-cli] Replace pubkey_of_signer with pubkey_from_source

[samkim-crypto]

Problem

The pubkey_of_signer function from solana-clap-v3-utils expects arguments to be parsed as String. However, if when using the more up-to-date way of declaring arguments using SignerSourceParserBuilder, clap expects arguments to be parsed directly as SignerSource rather than String and hence returns a downcast error when pubkey_of_signer is invoked.

Summary of changes

Replace pubkey_of_signer with pubkey_from_source.

Unfortunately, this way of parsing is a bit more verbose than the original code, but it should do the job. Perhaps an analogous function to pubkey_of_signer, which parses arguments directly as SignerSource can be added to solana-clap-v3-utils later on to make the code less verbose.

Closes #6623.

[buffalojoec]

This seems to work! Too bad it's so verbose now, but I agree some tuning in the Clap v3 utils crate could make this a bit more slick!