Adds custom inspect function to PublicKey

[stellaw1]

Problem

Public Keys are logged in a not-so-great format when console logged in Node.Js.

Example:

//   authorizedPubkey: PublicKey {
//     _bn: <BN: 919981a5497e8f85c805547439ae59f607ea625b86b1138ea6e41a68ab8ee038>
//   },

Summary of Changes

• Added a custom inspect function to PublicKey that returns its string format
• Added tests for the custom inspect function

Fixes #20022

[stellaw1]

Blocked by Flow - the custom inspect function that overrides console.log uses computed property names which is an ES6 feature that is not yet supported by the flow library (seen here).