Add KeepAliveMode and SupportedWebSocketSubProtocols options

[Shane32]

Adds a 'secure' keep-alive mode where:

• A ping is sent on a timer from the last received pong
• Ping messages carry a unique payload (random guid)
• Received pong messages verify the payload
• If a pong is not received with the correctly id in a timely fashion, the client is forcibly disconnected

Reason:

• With subscriptions that continuously send lots of data, there is no way to determine if socket backpressure due to slow client connections is causing resource exhaustion. This feature will validate that buffers are drained over the specified time period, as the client would not know what id to send with a pong if it did not process the incoming messages fast enough.

Consider:

• Name of KeepAliveMode enum values
• Adding separate timer property that represents the amount of time to wait for a corresponding pong after a sent ping. Currently this just matches the ping timer.

[github-actions]

Coverage Report

Totals
Statements:	94.8% ( 1933 / 2039 )
Methods:	81.42% ( 298 / 366 )

[coveralls]

Pull Request Test Coverage Report for Build 11541224015

Details

• 60 of 103 (58.25%) changed or added relevant lines in 6 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.9%) to 92.691%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/GraphQL.AspNetCore3/WebSockets/GraphQLWebSocketOptions.cs	1	2	50.0%
src/GraphQL.AspNetCore3/WebSockets/SubscriptionsTransportWs/SubscriptionServer.cs	7	8	87.5%
src/GraphQL.AspNetCore3/WebSockets/BaseSubscriptionServer.cs	22	27	81.48%
src/GraphQL.AspNetCore3/WebSockets/GraphQLWs/SubscriptionServer.cs	28	64	43.75%

Files with Coverage Reduction	New Missed Lines	%
src/GraphQL.AspNetCore3/GraphQLHttpMiddleware.cs	4	94.24%

Totals
Change from base Build 11541128763:	-1.9%
Covered Lines:	1933
Relevant Lines:	2039

---

💛 - Coveralls