libstore(filetransfer): Support store-specific http-version setting

[pkpbynum]

Motivation

I'd like to allow more flexibility in selecting the HTTP version used for a specific store.

This PR refactors the logic for selecting the HTTP versions implemented by libcurl, which is currently controlled by the global http2 setting. Instead we add an http-version setting to the HTTP binary cache store. In addition to allowing experimentation with new protocols, this allows different stores to implement different HTTP versions.

Backwards compatibility
When the store setting is http-version == none (the default), the global http2 setting is checked:

• if http2 == true (default) -> use CURL_HTTP_VERSION_NONE -- this differs from previous behavior using CURL_HTTP_VERSION_2TLS, and lets curl decide the best version to use based on the scheme (http/https) and handshake.
• if http2 == false -> use CURL_HTTP_VERSION_1_1 (matching previous behavior)

TLDR; users that have explicitly set http2 == false will experience no change. The default behavior, however, is now using CURL_HTTP_VERSION_NONE rather than CURL_HTTP_VERSION_TLS.

One might also argue for the deprecation of the http2 setting at this point, and just let folks override directly with http-version=http1.1 if they want it.

Context

Some brief discussion around QUIC here.

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[xokdvium]

Making this per-store seems nice, but exposing all the internal bits of curl seems like a step in the wrong direction. Wouldn't H11/H2/H3 be enough?

[pkpbynum]

Making this per-store seems nice, but exposing all the internal bits of curl seems like a step in the wrong direction. Wouldn't H11/H2/H3 be enough?

Yup sure, I just added them here for completeness but happy to trim it down. Although I am specifically interested in http2-prior-knowledge. We could also consider CURL_HTTP_VERSION_NONE to let curl handle the upgrades dynamically.

Also, reverted to draft while I figure out why it's not building on Linux.

[pkpbynum]

I updated this PR to compile properly on Linux. I had to copy-paste some BaseSettings templates--including the generic implementation in configuration.hh and abstract-settings-to-json.hh caused issues with ExternalBuilders. I'm not sure how to make this better.

@xokdvium I also trimmed down the allowed versions, and change the names to match exactly the CLI flags afforded by curl, e.g. --http1.1, --http2, --http2-prior-knowledge, --http3 ==> becomes http1.1, http2, http2-prior-knowledge, http3 as http-version settings