Feature/b2

[grisumbras]

Changelog: Feature: Adds a B2Deps and B2Toolchain generators for the B2 build system.
Docs: https://github.com/conan-io/docs/pull/XXXX

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[memsharded]

This has been opened a few hours later than #13592.
It would be great if you and @grafikrobot can talk and converge in one proposal

[grafikrobot]

@memsharded (and @grisumbras) yes, fully aware. I'm working on taking into account some things in this PR into the B2Deps one. Though I think splitting into two PRs is best. One for B2Deps and another for B2Toolchain. @grisumbras can do the B2Toolchain one with the B2Deps one as a basis.

[grisumbras]

Yes, don't review this. It is just for easy reference to what I've already done.

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