Original license headers are missing for context package.

[Oberon00]

In the PR description of #1658:

The API is heavily inspired by gRPC and Armeria. Much code / docs are borrowed too and I kept license headers in tact to reflect that.

which is sensible and actually required by the OTel/CNCF policy:

According to CNCF recommendations if you are contributing third-party code you will need to retain the original copyright notice.

-- https://github.com/open-telemetry/community/blob/master/CONTRIBUTING.md#copyright-notices

However, the statement from the PR description is unfortunately, wrong there is only the OTel license. Maybe the original license header got killed by spotlessApply.

CC @anuraaga

[Oberon00]

As it is now, e.g. the PersistentHashArrayMappedTrie.java file has no indication whatsoever that it is a 1:1 copy & paste from grpc-context. Knowing that would have eased my mind about the battle-testedness of this rather complicated code considerably 😃

[iNikem]

Spotless should be taught to accept two licenses in the same file. I think it has problems with that atm. Both here and in the instrumentation repository.

[anuraaga]

@iNikem I came up with a workaround for it but think I didn't merge that into the branch, let me show it in a bit :)