Update ChainmonstersRewards contract to allow reward metadata updates

[maggo]

• Adds pub fun updateRewardMetadata(rewardID: UInt32, metadata: String) on the Admin resource
• Changes pub let metadata: String to pub var metadata: String and adds access(contract) fun setMetadata(metadata: String) on the Reward to make the metadata updateable
• Adds update_reward_metadata.cdc transaction as an example

Also converted all indentation to spaces and removed trailing whitespace for readability ;)

For better diff:
https://github.com/bsidegames/chainmonsters-smart-contracts/pull/5/files?w=1