[otlp] update protobuf definitions to new location/version in datadog-agent repo

[truthbk]

What does this PR do?

This PR updates the protobuf imports such that the new definitions in pkg/proto in the agent repository are used. This includes some changes due to the gogoproto nullable type being removed from the definitions.

Additionally, due to a chicken-egg problem, these changes are currently pointing to a WIP PR on the datadog-agent repository (PR to be posted soon). Unfortunately the current protobuf structure (definitions part of https://github.com/DataDog/datadog-agent) doesn't make it easy to make these updates.

Motivation

Refactor of protobuf definitions in https://github.com/DataDog/datadog-agent so that all definitions now live in pkg/proto as opposed to scattered all over the repository.

[songy23]

I verified make gen-licenses passed locally but failed on CI, #104 to follow up

[songy23]

This is the only change needed in this file, please revert other changes.

[gbbr]

Can we revert this change? This package doesn't even exist on main (https://github.com/DataDog/datadog-agent/tree/main/pkg/proto/pbgo), and we can't update the exporter anymore to use this code because it depends on pkg/trace from the datadog-agent, which isn't updated. So we are technically blocked.

We can keep the PR open for when it's ready but I don't think we should have merged it.

[songy23]

I believe this needs to be merged to break a circular dependency issue (?) in Agent because @truthbk needs a pseudo version of mapping go with this change and use it in Agent. But if this is blocking the upgrade in datadog exporter, I agree we should revert it for now.

[gbbr]

Sure, but this can be referenced elsewhere as a commit from this (unmerged) PR so it should be fine for Jaime (I think?)