Toolchain refactoring

[chancila]

This is kind of a big change. Key takeaways:

• Use a single toolchain type, having 3 for 3 different tools is unnecessary, especially since everything is downloaded together
• Expose the toolchain using a provider (BufToolchainInfo), this is how toolchains typically are used
• Refactor toolchain download to prepare toolchains for all execution platforms, due to how toolchain lazy loading works the actual toolchain files are not fetched until it's needed, registering a toolchain does not cause all of it's attributes to be evaluated (namely toolchain.toolchain which contains the ToolchainInfo with the BufToolchainInfo)
• Due to the above, it's possible to run buf rules in different execution platforms for host (ie if using remote execution)

[CLAassistant]

All committers have signed the CLA.

[chancila]

Are there any tests for validating these rules? there seem to not be

[srikrsna-buf]

Hey @chancila! If you can sign the CLA, I'll take up https://github.com/bufbuild/rules_buf/pull/26#discussion_r949296569 and resolve the merge conflicts to get this merged.

[chancila]

thanks, signed