api: support disable ALPN in CTP

[guydc]

What type of PR is this?

What this PR does / why we need it:

• Makes it possible to opt-out of ALPN in CTP by specifying an empty (non-nil) list of ALPN protos.
• Improves ALPN docs in tls_types.go relating to default, downstream/upstream behavior, etc.

Which issue(s) this PR fixes:

Fixes #4462

Release Notes: Yes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4515   +/-   ##
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  Coverage        ?   65.55%           
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  Files           ?      211           
  Lines           ?    31945           
  Branches        ?        0           
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  Hits            ?    20943           
  Misses          ?     9757           
  Partials        ?     1245           

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[arkodg]

LGTM thanks !