Initial partial support for C++20 in Envoy

*Initial partial support for C++20 in Envoy*

*Description*:

Context:
The latest standard version ratified and published by ISO in December of 2020 as ISO/IEC 14882:2020 (informally known as C++20) —a very significant update over C++17—is widely (but not yet fully) supported by major compilers such as gcc-10 and clang-10.
C++20 is already being used as the standard version by some companies and this number will continue to increase as time passes, so an optional C++20 compatible version of Envoy would be needed.

What we're proposing:
Supporting C++20 by making the necessary changes, implementing the new features for better compatibility, and replacing deprecated features. We will be making these changes in a **modular way** as to not make the change so abrupt. We'll be starting with "unpopular" modules (those which are not widely used) and modules which require just a few changes to be fully compatible with C++20.

- Ideas & suggestions on how to split up the PRs would be very helpful. Thanks.