Set better default timeouts for API exchanges in DesktopAgentProxy and allow DesktopAgents to control them

[kriswest]

Enhancement Request

As discussed at meeting #1478, the DesktopAgentProxy implementation used by getAgent to communicate with bowser-based Desktop Agents currently uses 10 second timeout for all API calls. This is appropriate for the majority of API calls, but problematic for open, raiseIntent and raiseIntentForContext, which do not return responses until an application has been launched and has connected to the Desktop Agent, allowing it to deliver context.

The FDC3 Standard defines a minimum timeout of 15 seconds that MUST be allowed for apps to launch and reconnect, but also defines no upper bound, as that is determined by the Desktop Agent.

The DesktopAgentProxy should therefore support different timeouts for API calls that launch applications and should allow DesktopAgents that it connects to (via getAgent) to control the timeouts (as the Desktop Agent is more likely to know the upperbound that it allows internally or that apps within its scope require).

This issues proposes to retain the default timeout of 10s for most API calls, to extend the default timeout for API calls that open apps (open, raiseIntent and raiseIntentForContext) to 100s and to allow Desktop Agents to specify custom timeouts for the two classes of API call. Custom timeouts would need to be sent as part of the WCP connection messages, in either WCP3Handshake or WCP5ValidateAppIdentityResponse.