multiple example configs for different relayer use cases

[anthony-near]

https://github.com/near/pagoda-relayer-rs/issues/26

• [x] example config for basic relayer with contract whitelist
• [x] using redis for keeping track of allowances
• [x] fastauth config
• [x] pay with FT config (similar to SWEAT relayer)
• [x] add optional payload check for FT transfer to burn address to config
• [x] whitelisted sender addresses config
• [x] whitelisted sender addresses (delegate_action_receiver_ids) decoupled from fastauth in code
• [x] add capital efficiency note to README