make tzdb an optional dependency

[davidkna]

This improves the minimum supported rust version a bit, and can be used in cases where having access to the local timezone is less important.

[baoyachi]

@davidkna Thx. It's cool

[baoyachi]

@davidkna
I want tell you add tzdb crate context. I tried the method you provided before, but the run MacOS returned error, I found that tzdb solved this problem, so I used tzdb at present, the timeliness of TZDB will be more accurate.

So，I add some description in code .

+ // Warning: Attempt to create a new OffsetDateTime with the current date and time in the local offset. If the offset cannot be determined, an error is returned.
+// At present, using it on MacOS return error. Use it with careful.
+// Suggestion use feature tzdb crate exposed function at below.

shadow-rs/src/data_time.rs

Lines 55 to 57 in 1ce70bb

	// Warning: Attempt to create a new OffsetDateTime with the current date and time in the local offset. If the offset cannot be determined, an error is returned.
	// At present, using it on MacOS return error. Use it with careful.
	// Suggestion use feature tzdb crate exposed function at below.

I can open this function, but developers need to know this problem.