cmd/go: add GOAUTH support for TLS client certificates

[cazlo]

GOAUTH can provide HTTP request headers, but it cannot configure client
certificates needed during TLS handshakes.

Add a built-in mtls method that associates PEM certificate and private
key files with one canonical HTTPS origin. Load the files only for
matching requests and use a distinct cloned transport for each
configuration so redirects re-evaluate certificate selection.

Reject mTLS when GOINSECURE applies or when an HTTPS proxy is selected.
Keep normal server certificate verification unchanged. Add unit and
script tests covering origin matching, redirects, lazy loading,
combined and separate PEM files, go-import discovery, and HTTPS
GOPROXY use.

For #30119

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