TlsAcceptCallbacks support for rustls backend

[dpfeifer2]

Support TlsAcceptCallbacks for the rustls backend

Ports the TlsAcceptCallbacks support from upstream #833, excluding the parts flagged in review there: that PR bundles unrelated changes (intentional webpki policy bypass on cert loading, custom ServerCertVerifier support in the connector) which @nojima asked to split into a separate PR (#833 (comment)) for independent security discussion. This PR takes only the callbacks feature.

Changes
• TlsSettings::with_callbacks(cb) now works with rustls (previously returned an error); added set_certificate_chain_file() / set_private_key_file() and set_cert_resolver() for dynamic cert selection
• TlsRef extended from an empty struct to expose post-handshake state.
• handshake_with_callback() passes a populated TlsRef to handshake_complete_callback, matching the OpenSSL/BoringSSL behavior
• Added test_handshake_complete_callback integration test

Deviations from #833
• No webpki bypass: cert/key loading still goes through with_single_cert()
• Test uses the ring provider instead of aws_lc_rs

[dpfeifer2]

Hey @nojima , @johnhurt and @jsulmont , not much seems to have happened regarding #833 for a while. I would like to get the TlsAcceptCallbacks support for rustls. Given the change of #833 seems to have already been reviewed is there any chance we can get this PR merged (with the proposed split to leave out any unrelated changes from #833). Would appreciate it! Let me know if you have further comments I should address on this PR.
CC @stepancheg fyi

[jsulmont]

Author of #833 here — this is the right split, thanks for picking it up @dpfeifer2, and sorry for letting mine stall. I got dragged into other work, and on our end it's since been decided to stick with the OpenSSL route for now.

The TlsAcceptCallbacks support is the uncontroversial core, and keeping cert/key loading on with_single_cert() with set_cert_resolver() as the opt-in escape hatch is exactly what @nojima asked for: the default webpki behavior is untouched. I'm closing #833 in favor of this. LGTM.

[nojima]

I'll take a look at this PR over the weekend.