Renaming to otel-instrument and default configs

[srprash]

As a follow up to the https://github.com/aws-observability/aws-otel-java-instrumentation/pull/953 and from internal discussion with @wangzlei:

• Renaming otel-handler to otel-instrument to be consistent with other ADOT lambda layer script naming.
• Enabling Application Signals by default with the following environment variables already set. Note that users can still override these environment variables if they want to.

  AWS_LAMBDA_EXEC_WRAPPER=/opt/otel-handler
  OTEL_AWS_APPLICATION_SIGNALS_ENABLED=true
  OTEL_LOGS_EXPORTER=none
  OTEL_METRICS_EXPORTER=none
  OTEL_TRACES_EXPORTER=otlp

ADOT Python implementation for reference: https://github.com/aws-observability/aws-otel-python-instrumentation/blame/main/lambda-layer/src/otel-instrument

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