Add CHACHA ciphers for TLS 1.2

[chriscasano]

Please add support for the following ciphers in TLS 1.2

• TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
• TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256

Jira issue: CRDB-15593

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[bdarnell]

Note that these ciphers are already supported, but they're listed in our source code under the non-standard aliases TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305 and TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305. We should switch to the standard names to avoid confusion, but there is no functional change (and no need for a new release/upgrade to support these ciphers)