Switch to abseil::variant as default variant class across api

[lalitb]

opentelemetry-cpp api currenty brings 3 variants of variant class switchable at build time.

1. std variant ( HAVE_CPP_STDLIB switch : https://github.com/open-telemetry/opentelemetry-cpp/blob/main/api/include/opentelemetry/std/variant.h

2. abseil variant ( HAVE_ABSEIL_VARIANT switch: https://github.com/open-telemetry/opentelemetry-cpp/blob/main/api/include/opentelemetry//nostd/absl/types/variant.h )

3. mark variant ( enabled by default: https://github.com/open-telemetry/opentelemetry-cpp/blob/main/api/include/opentelemetry/nostd/mpark/variant.h )

Going forward - it would be better to converge to single variant class ( possibly abseil::variant as it is usable with vc2015 ), and get rid of rest of the libraries.

Refer to discussions in this PR thread: #565