Adding support to provide start date time for cosmos feed start

[adityavaish]

Exposing StartTime of ChangeFeedProcessorOptions to client attribute so that a client to specify the start point of the feed. The date time should ideally be in UTC but it can take any format that can be parsed by C# DateTime

Added default test case to check the added attribute.

[net-foundation-cla]

All CLA requirements met.

[ealsur]

@brettsam Do we have any other trigger that passes datetime information? What is the recommendation on the format?

[adityavaish]

A gentle ping on the review!

[adityavaish]

@fabiocav @brettsam - please help review

[brettsam]

@brettsam Do we have any other trigger that passes datetime information? What is the recommendation on the format?

I don't think we do -- I was just trying to think through that as well. I know we have some that use TimeSpan, let me double-check how we handle that.

[ealsur]

My concern is more around other languages, if the ISO format would work fine for them or there is a different one. Otherwise I see no blockers on the PR other than the test gap at this point.

[brettsam]

I have one other comment that I can't directly link to (and maybe it can be an immediate follow-up PR by @ealsur?) -- but now that the parsing of attribute strings can have some interesting side effects (time zones...), it may be good to explicitly log the fully-resolved ProcessorOptions with a _logger.LogDebug(...) statement here: https://github.com/Azure/azure-webjobs-sdk-extensions/blob/dev/src/WebJobs.Extensions.CosmosDB/Trigger/CosmosDBTriggerListener.cs#L129. That would help immensely when trying to support folks that may have configured something slightly incorrectly.