Accept an optional TimeZoneId as an argument to NowDate

[pstephenson02]

This is a very naïve attempt to give users the ability to customize the timezone when using the NowDate filter. I had a customer request this feature for use with the Email notification step - https://octopus.zendesk.com/agent/tickets/64730
And we had some brief discussion in Slack: https://octopusdeploy.slack.com/archives/C033W4273/p1612912152089000

I'm opening this as a draft primarily to help drive discussion. I'm pretty sure that this implementation would break down and not work for chained constructions like:

#{ | NowDate "Eastern Standard Time" | Format Date "MM/dd/yy H:mm:ss"}

where the Format Date filter is not aware that the previous string was based on some non system local time zone.
As far as I can tell, there doesn't appear to be any kind of state or passed context for the entire expression where something like this timezone could live and be referenced, and doing something like that feels like a substantial change to the library.

Interested to hear people's thoughts. Thanks!