fix: mock backend to return the example value provided in date and time v3 matchers

[pulak777]

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Currently while using MatchersV3 date and time, mock backend returns current date instead of the provided example date.

This works correctly for datetime. So this pr applies similar fix to what was done in datetime (removing pact:generator:type from output object when example value is present)

Added some tests in pact-reference for this scenario as well - pact-foundation/pact-reference#489

[pulak777]

Looks like there's been a discussion on this before - #1076

[mefellows]

Thanks!

[pulak777]

Thank you @mefellows. From this comment and the commit where the example was made mandatory, it looks like we can make the example input optional. As rust ffi generates example value from the given regex (which probably was not the case then)