Extract Utf8Parser date helpers to static helper class

[layomia]

The DateTime(Offset) parsing logic in Utf8JsonReader (dotnet/corefx#35903) depends on copied implementations of these helpers. Extracting them would remove this duplication.

[karelz]

@layomia please only 1 area label - here's why: https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/issue-guide.md#triage-rules---simplified
@joshfree FYI we do not use assignments, unless people are actively working on issues ... (see above)

[joshfree]

Marking this as up-for-grabs

[ViktorHofer]

@layomia I know about people being interested in helping out with up-for-grabs issues. Can you please check if this is still todo and if yes provide some basic steps how to tackle the issue? Thanks