Start rewriting TLS extension handling.

Introduce a TLS extension handling framework that has per-extension type
functions to determine if an extension is needed, to build the extension
data and parse the extension data. This is somewhat analogous to BoringSSL,
however these build and parse functions are intentionally symetrical. The
framework is hooked into the existing TLS handling code in such a way that
we can gradual convert the extension handling code.

Convert the TLS Server Name Indication extension to the new framework,
while rewriting it to use CBB/CBS and be more strict in the process.

Discussed with beck@

ok inoguchi@

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