Don't make users install protoc

[avantgardnerio]

Which issue does this PR close?

Closes#3947.

Rationale for this change

Described in issue.

What changes are included in this PR?

1. have build.rs download and extract protoc
2. remove all references to installing protoc in the base OS

Are there any user-facing changes?

No 🤞

[tustvold]

An alternative that might be less potentially problematic, as there are reasons prost stopped vending protoc binaries, might be to vend the generated descriptor set. I experimented with doing this for pbjson-types in influxdata/pbjson#79.

Another option might just be to vend the generated code, perhaps using something like https://docs.rs/protoc-gen-prost/latest/protoc_gen_prost/

Edit: I will have a play with the latter option today

[avantgardnerio]

there are reasons prost stopped vending protoc

@tustvold skimming the reasons, it looks like they are down to user control. That made me realize I was overriding user choice. I added new code that does a no-op if the user has already set the PROTOC env var to their compiler of choice... does that resolve your concerns with this PR?

[tustvold]

That's part of it, the other part is a lack of desire to maintain code that acts as a DIY package manager. The auto-generation of code for prost seems to just be a large source of issues, I'm inclined to think we should just check in the generated sources, like we do for thrift or flatbuffers, and move on 😅

[avantgardnerio]

Putting this down as a historical PSA: there is more discussion in the alternative PR.

[avantgardnerio]

Closing in favor of #3947