Add keep-alive mode to ensure pings/pongs are received timely

[Shane32]

• Need to consider new options and behavior
• Should provide option to validate that pongs are received under graphql-ws
• Or maybe should provide option to validate that any packet has been received
• Need to consider both protocols

Idea (for graphql-ws):

• Add option/mode to ensure any packet has been received within a specified interval
• During ping interval, check when last packet rec'd was, and if too long, send ping

Idea (for graphql-ws):

• Add option to ensure that pongs respond to pings
• Pings send unique identifier (maybe a timestamp for simplicity)
• Pongs verify that unique identifier is received within the specified time
• Note: may not work with client libs if they don't mirror the ping payload

Notes for subscription-transport-ws:

• 'ka' packet is not a client->server packet
• 'ka' packets received by the client does not trigger a response
• Maybe need to be able to disable this protocol to enforce keep-alive behavior