[CC-6097] amount on tokenization validation

[vincentleandr]

• Add logic to check for amount for single and multiple use token
• Make amount optional for tokenization
• Update readme file

Test

Single use with proper amount

Single use with 0 amount

Single use with no amount inputted

Multiple use with proper amount

Multiple use with 0 amount

Multiple use with amount less than 0

Multiple use with no amount inputted

[javiersuweijie]

@vincentleandr please help to bump the version here https://github.com/xendit/xendit-sdk-ios-src/blob/master/Xendit.podspec#L3

else lgtm