Update mpl-core, Pubkey serde serialization via DisplayFromStr

[danenbm]

This will make Pubkeys in mpl-core plugins print as base58 strings rather than arrays, i.e.:

RuleSet::ProgramAllowList JSON:

{"ProgramAllowList" ["8bAnE38uhVdTzK2Z8FBi9Gm2C1wrJo6AS2G9dfMqVg44","FUDsxraf8C3e6N9KcoVqqhKSm6MiBd8UveF452jxDqX8"]}

UpdateDelegate JSON:

{"additional_delegates":["FUDsxraf8C3e6N9KcoVqqhKSm6MiBd8UveF452jxDqX8"]}