Subscription connection parameters support

[HaraldNordgren]

Adds support for subscription (websocket) connection parameters support.

We are having an issue with authorization when using subscriptions in genqlient. Our mobile app team is using Apollo for GraphQL requests, and Apollo seems to sends its auth in a way that is different from how genqlient does it:

https://www.apollographql.com/docs/react/data/subscriptions#5-authenticate-over-websocket-optional

This is causing a problems, because we use genqlient for testing the backend. It seems that the way Apollo is implemented, authentication (and other headers) are not actually sent as HTTP headers, but as connection parameters instead.

I have:

• Written a clear PR title and description (above)
• Signed the Khan Academy CLA
• Added tests covering my changes, if applicable
• Included a link to the issue fixed, if applicable
• Included documentation, for new features
• Added an entry to the changelog

[HaraldNordgren]

Since there was never a a release that includes NewClientUsingWebSocket, we should be free to update the signature if we want, and possible only have one function:

func NewClientUsingWebSocket(endpoint string, wsDialer Dialer, headers http.Header, connParams map[string]any) WebSocketClient

[HaraldNordgren]

A big gotcha regarding HTTP headers and connection parameters, that I realized recently, is that websocket requests in 99desgins gqlgen enter a different path, going through here:

	gqlgenServer.AddTransport(transport.Websocket{
		InitFunc: func(ctx context.Context, initPayload transport.InitPayload) (context.Context, *transport.InitPayload, error) {
			// initPayload contains connection parameters
		},
	})

than the middleware path that would normally be expected:

	r.With(ah.VerifyAuthMiddleware).Method(http.MethodPost, "/v1", c.GraphServer)
	r.With(ah.VerifyAuthMiddleware).Method(http.MethodGet, "/v1", c.GraphServer)

[HaraldNordgren]

Ping @benjaminjkraft and @matthieu4294967296moineau, do you think it would be possible to get a quick look here? 🤗