tsnet: base HTTPClient's transport on http.DefaultTransport

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Previously Server.HTTPClient returned a client whose transport set
only DialContext, losing http.DefaultTransport's defaults such as
ForceAttemptHTTP2, MaxIdleConns, IdleConnTimeout, TLSHandshakeTimeout,
and ExpectContinueTimeout. Now it clones http.DefaultTransport and
overrides DialContext to dial over the tailnet, so it picks up those
settings and any future defaults.

Proxy is explicitly nil: an HTTP proxy from the environment would be
dialed through the tailnet, where it's unlikely to be reachable.

The new test compares the transport field by field against a clone of
http.DefaultTransport and fails on any unknown future field, forcing a
decision about how HTTPClient should handle it.

Updates tailscale/corp#47393 (as motivation)

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I1e800982ee11d386699dd4f588c1fd2ce392f229

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