* improve bot token validation by trying to decode user id from token

Try to decode the user id from the supplied bot token as a way of validating the token. If this should fail, indicate that the token is invalid.

* Update the tokenutils tests to pass the new validation checks

* Add test case for CheckBotTokenValidity method

* lint: clean up whitespace

* Add check for null or whitespace string, lint whitespace

* fix userid conversion

* Add hint to user to check that token is not an oauth client secret

* Catch exception that can be thrown by GetString

* Refactor token conversion logic into it's own testable method

* Update the minimum bot token length to 58 char

- Updates the minimum length of a bot token to be 58 characters. An older 58 char bot token was found by Moiph
- Makes this value an internal const instead of a magic number

* update the TokenUtils tests for 58 char min

* Add input validation for bot tokens based on their length

* Add token validation to BaseDiscordClient#LoginAsync

Adds a TokenUtils class which is used to validate that tokens are correct

* Revert changes to DiscordRestApiClient

* Add Unit tests to the TokenUtils class, fix a logic error that was caught by those tests

* Allow for API to throw exceptions

Moves the validation of tokens to be inside of LoginInternalAsync, and writes a Warning to the console when the supplied tokens are invalid