Investigate Boost license implications

[maxgolov]

A portion of nostd::variant currently requires a Boost-licensed header.
This may be an issue for some commercial products.

Two alternatives to consider:

• research if we can get that portion code contributed under Open Telemetry (Apache-style) license. That way we still satisfy the ABI stability guarantees.

• the other alternative is to compile with std Ability to use Standard Library instead of opentelemetry nostd classes #67 . That approach means that commercial products building from source with standard library cannot safely interop with other prebuilt Open Telemetry modules.

[maxgolov]

I'll try to remove MPark variant and replace by Abseil Variant absl::variant. There is no other code in this repo that requires Boost License.

[maxgolov]

I would propose we totally remove MPark variant - replacing it with a copy of implementation borrowed from absl::variant. I need this anyways to start supporting Visual Studio 2015 #314 ... Thus, removing MPark variant we will get rid of the Boost licensing issue.

[maxgolov]

Sorry for inactivity on it -- I'll switch to absl::variant #572 thus removing the need for Boost License. ETA: April 2021

[maxgolov]

Closing as Duplicate of #572 - switching to Abseil Variant would allow us to remove Boost -Licensed code.