Introduces Profiling Data Model v2 (open-telemetry/oteps#239)

This is second version of the Profiling Data Model OTEP. After [we've
gotten feedback from the greater OTel
community](open-telemetry/oteps#237) we went
back to the drawing board and came up with a new version of the data
model. The main difference between the two versions is that the new
version is more similar to the original pprof format, which makes it
easier to understand and implement. It also has better performance
characteristics. We've also incorporated a lot of the feedback we've
gotten on the first PR into this OTEP.

Some minor details about the data model are still being discussed and
will be flushed out in the future OTEPs. We intend to finalize these
details after doing experiments with early versions of working client +
collector + backend implementations and getting feedback from the
community. The goal of this OTEP is to provide a solid foundation for
these experiments.

So far we've done a number of things to validate it:
* we've written a new profiles proto described in this OTEP
* we've documented decisions made along the way in a [decision
log](https://github.com/open-telemetry/opentelemetry-proto-profile/blob/main/opentelemetry/proto/profiles/v1/decision-log.md)
* we've done benchmarking to refine the data representation (see
Benchmarking section in a [collector
PR](petethepig/opentelemetry-collector#1))

* diff between original pprof and the new proto:
[link](open-telemetry/opentelemetry-proto-profile@2cf711b...petethepig:opentelemetry-proto:pprof-experiments#diff-9cb689ea05ecfd2edffc39869eca3282a3f2f45a8e1aa21624b452fa5362d1d2)

We're seeking feedback and hoping to get this approved. 

---

For (a lot) more details, see:
* [OTel Profiling SIG Meeting
Notes](https://docs.google.com/document/d/19UqPPPlGE83N37MhS93uRlxsP1_wGxQ33Qv6CDHaEp0/edit)

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Co-authored-by: Juraci Paixão Kröhling <[email protected]>
Co-authored-by: Christos Kalkanis <[email protected]>
Co-authored-by: Felix Geisendörfer <[email protected]>
Co-authored-by: Reiley Yang <[email protected]>