Plugin: Add Simple Keychain Implementation

[iMacHumphries]

This should really be extended to allow further configuration/control of the keychain interface, but for now this solves the problem I'm having and I wanted to share.

Example usage:

var keychain = null
func _ready():
    if Engine.has_singleton("Keychain"):
        keychain = Engine.get_singleton("Keychain")
        
        keychain.set_string("username", "knight123")
        var username = keychain.get_string("username")
        print("Stored username: ", username)
        keychain.delete_key("username")

Test plan

        class_name Keychain extends node # Singleton
        
        extends Node

var _keychain

func _ready() -> void:
	if Engine.has_singleton("Keychain"):
		_keychain = Engine.get_singleton("Keychain")

func set_string(key: String, value: String) -> void:
	if _keychain:
		_keychain.set_string(key, value)

func get_string(key: String) -> String:
	if _keychain:
		return _keychain.get_string(key)

func delete_key(key: String) -> void:
	if _keychain:
		_keychain.delete_key(key)
        
        #################
        
	print("Keychain test")
	var result = Keychain.get_string("test")
	print("1. Get test:", result)
	if not result:
		Keychain.set_string("test", "1")
		print("2. Save test:", Keychain.get_string("test"))

1. Run on new application. Result:
   Keychain test

2. Get test:

3. save test: 1

4. Delete app from iPhone and run again. Result:
   Keychain test

5. Get test: 1

[jackchapm]

+1, hoping to bring some attention to this PR as it seems like a common enough task that a first party plugin should be made for this