arrow

GH-32662: [C#] Make dictionaries in file and memory implementations work correctly and support integration tests (#39146)

### Rationale for this change

While dictionary support was implemented for C# in #6870 for streams, support did not extend to files or memory buffers. This change rectifies that.

### What changes are included in this PR?

Changes to the memory and file implementations to support reading and writing of dictionaries, including nested dictionaries.
Changes to the integration tests so that they work with dictionaries.
Enabling the dictionary tests in CI.

### Are these changes tested?

Yes, both directly and indirectly via the integration tests.

### Are there any user-facing changes?

No.

* Closes: #32662

Authored-by: Curt Hagenlocher <curt@hagenlocher.org>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>