Add GetStringView like GetString, GetCstring (#5937)

Add GetStringView like GetString, GetCstring (#5937)

* Added missing EndTable() call to VerifyObject()

VerifyObject called VerifyTableStart() but not EndTable(). This made Verifier::VerifyComplexity() increase depth_ with each table, not with the depth of tables.

https://groups.google.com/forum/#!topic/flatbuffers/OpxtW5UFAdg

* Added Check to VerifyAlignment

https://stackoverflow.com/questions/59376308/flatbuffers-verifier-returns-false-without-any-assertion-flatbuffers-debug-veri

* Add GetStringView (Convenience function to get string_view from a String returning an empty string_view on null pointer) like GetString, GetCstring

Co-authored-by: stefan301 <Stefan.Felkel@de.Zuken.com>