feat: add null support to adaptive array builders (open-telemetry#583)

Part of open-telemetry#534

Support adding nulls to adaptive array builders.

This PR also fixes all the "unused" clippy warnings. Originally this
module had `#[allow(unused)]` on it to avoid unused code clippy warnings
until this was integrated with code to encode OTAP. It _should_ have had
`#[allow(dead_code)]`, because `unused` also controls clippy warnings
for unused imports and unused results, which we don't want.

After the next release of arrow, I'll come back and clean up the
`append_nulls` method in a few places where we're using loops when we
should just be able to call `append_nulls` on the underlying builder.
Some arrow builders don't support this in the current version, but was
added here: apache/arrow-rs#7606

Co-authored-by: Laurent Quérel <l.querel@f5.com>

Author: albertlockett

rust/otel-arrow-rust/src/encode/record.rs

@@ -1,6 +1,6 @@
 // Copyright The OpenTelemetry Authors
 // SPDX-License-Identifier: Apache-2.0
 
-// TODO remove allow(unused) when we use what's in this module to encode OTAP
-#[allow(unused)]
+// TODO remove allow(dead_code) when we use what's in this module to encode OTAP
+#[allow(dead_code)]
 mod array;