Implement secure storage

[jonathanpalma]

Implement a secure storage library and replace the current implementation of how tokens are stored

btc-payer/src/providers/AuthProvider.tsx

Lines 52 to 53 in 9a16b46

	// TODO: retrieve token from keychain/encrypted-storage
	AsyncStorage.getItem(TOKEN),

btc-payer/src/providers/AuthProvider.tsx

Lines 71 to 72 in 9a16b46

	// TODO: store token in keychain/encrypted-storage
	AsyncStorage.setItem(TOKEN, token),

Useful links

https://reactnative.dev/docs/security#storing-sensitive-info

https://reactnative.dev/docs/security#secure-storage

Potential libraries

https://github.com/emeraldsanto/react-native-encrypted-storage

https://github.com/oblador/react-native-keychain