Add support for OTEL extensions

[dmikusa]

Describe the Enhancement

Paketo has a format for customizations and extensions, which you can see in the New Relic & Apache Tomcat buildpacks. We want to do the same thing for the OpenTelemetry buildpack so that users can include custom extensions.

Possible Solution

Follow the same pattern that's used in New Relic & Apache Tomcat buildpacks for extensions.

Motivation

See https://github.com/orgs/paketo-buildpacks/discussions/315#discussioncomment-11392876