Allow non-default parameter `utcNow()` calls

**Is your feature request related to a problem? Please describe.**

`utcNow()` as per the documentation can only be assigned as the default value of a parameter. This means if the sole function of using `utcNow()` is to calculate some time `T` in the future (for example for SAS token expiry) then a parameter is being exposed when it ought not to be.

**Describe the solution you'd like**

Allow `utcNow()` to be called anywhere inside the template and/or `templateStartTime()` (or better-named) to be called anywhere inside the template which has the time the instant the template was invoked (what `utcNow()` as a default parameter does now, but while keeping `utcNow()` unchanged).

Start by reviewing the documented behavior of utcNow() and the requested templateStartTime() alternative. Clarify whether calls should be allowed anywhere, how invocation time differs from current time, and what naming and scoping are acceptable; done means future-time calculations such as SAS token expiry need no exposed parameter.

Written by the indexing model from the issue text.