feat(HTTPConfig): expose config to force-attempt HTTP/2

[francoposa]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Added flag to pass through ForceAttemptHTTP2 - we are experiencing extremely high tail latencies with GCS that can be mitigated in part by HTTP/2, at least in our benchmarks so far.

This deals with the Go http.Transport's design choice of silently downgrading to HTTP/1.1 when a custom Transport is used:

	// ForceAttemptHTTP2 controls whether HTTP/2 is enabled when a non-zero
	// Dial, DialTLS, or DialContext func or TLSClientConfig is provided.
	// By default, use of any those fields conservatively disables HTTP/2.
	// To use a custom dialer or TLS config and still attempt HTTP/2
	// upgrades, set this to true.
	ForceAttemptHTTP2 [bool](https://pkg.go.dev/builtin#bool)

Verification

Test are added for config parsing & validation - we verified the performance difference in a benchmark here where we just hardcoded the flag in objstore/exthttp/transport.go.