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 ✅

Project coverage is 65.62%. Comparing base (0b69387) to head (b1bb1bc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4515   +/-   ##
=======================================
  Coverage   65.61%   65.62%           
=======================================
  Files         210      210           
  Lines       31622    31622           
=======================================
+ Hits        20750    20751    +1     
  Misses       9669     9669           
+ Partials     1203     1202    -1     

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