revert^2: experimental tls.external site setting
Created by: keegancsmith
reverting the revert https://github.com/sourcegraph/sourcegraph/pull/7741
AWS Code Commit broke with two commits: https://github.com/sourcegraph/sourcegraph/pull/7651 and https://github.com/sourcegraph/sourcegraph/pull/7648
The first broke it because previously NewCertOpt would not be run for it, now it always would be. The second broke it further because that PR can't wrap non-http.Transport RoundTrippers. This PR introduces a marker interface to let us skip that.
The effect is AWS Code Commit won't respect the new tls.external site setting. This is fine for now, since the setting is experimental.
Test Plan: Run e2e tests on enterprise dev.
Part of https://github.com/sourcegraph/sourcegraph/issues/71
Fixes https://github.com/sourcegraph/sourcegraph/issues/7745 Fixes https://github.com/sourcegraph/sourcegraph/issues/7744 Fixes https://github.com/sourcegraph/sourcegraph/issues/7738