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ARROW-7318: [C#] TimestampArray serialization failure

Fixes bug where FlatBuffer serialization of TimestampType was not serializing the time zone, resulting in the time zone not being preserved. Existing round-trip serialization tests did not catch this bug, because they were both not including a timestamp field in the test record batches, and did not include tests for comparing array types.

The test code was refactored to lift an abstraction from buffers to arrays; this both simplifies the code (we can use the fluent builder API) and offers more flexibility for comparing parametric array types like binary, string, timestamp, date/time, etc.

Closes #5965 from chutchinson/bug/1137-timestamp-timezone-serialization and squashes the following commits:

4b042678b <Christopher Hutchinson> simplified assertion
4f8120109 <Christopher Hutchinson> added missing license
f55a15f98 <Christopher Hutchinson> added ValueBuffer test assertions; added fluent builder for timestamp array; removed timezone conversion logic
70a4f3308 <Christopher Hutchinson> fixed TimestampArray flatbuffer serialization bug

Authored-by: Christopher Hutchinson <cshutchinson@fzcorp.com>
Signed-off-by: Eric Erhardt <eric.erhardt@microsoft.com>