Add exporter data model impl for profiling signal type.

[jhalliday]

Second step towards supporting the new experimental signal type for profiling...

Following on from the interfaces in https://github.com/open-telemetry/opentelemetry-java/pull/6374 this adds the AutoValue generated implementation classes. Less to discuss here, it's much more straightforward than the interfaces!

• Still no test coverage, since most of the code is auto generated anyhow. For other similar classes the test coverage is indirect via the Marshaler tests, which for these classes won't be along until the next PR. Meanwhile codecov will probably continue to sulk.

• The ByteBuffer use in ProfileContainerData presents a challenge. For other similar cases, the mutable object could be wrapped during the static create(...) call with e.g. Collections.unmodifiableList, such that the auto-generated getter can simply return the immutable wrapper. That pattern doesn't work with .asReadOnlyBuffer() since the resulting wrapper object is not immutable, though the content is. For now I've gone with adding an extra getter, as AutoValue won't support overriding the default one. The usability of the resulting API isn't great, but perhaps adequate as the only real use case is the Marshalers and they use it correctly. The alternative design is to disable AutoValue for that class and hand-tool all the getters, but that increases maintenance and cognitive load.