avro

AVRO-2784: Add the possibility to append to existing files in C# (#853)

*Implementation of Java AppendTo() as is

* API optimisation.

* optimisation.

* naming error fix

* Before unit tests.

* Unit tests draft

* unit tests

* Suppress CA1000 warnings for two new methods

Suppression performed to maintain compatibility

Co-authored-by: Volodymyr Aleksandrov <volodymyr.aleksandrov@pl.abb.com>