update fallbackCertificate design

[stevesloka]

Updates some additional concepts around how the fallbackCertificate is going to be enabled. The key changes are:

1. The fallbackSecret must be delegated via TLSCertificateDelegation
2. The minimum-protocol-version will be used from the Contour configuration file
3. Client Validation is not compatible with Fallback Certificate

Signed-off-by: Steve Sloka [email protected]

[codecov]

Codecov Report

Merging #2504 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2504   +/-   ##
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  Coverage   76.61%   76.61%           
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  Files          68       68           
  Lines        5513     5513           
=======================================
  Hits         4224     4224           
  Misses       1192     1192           
  Partials       97       97           

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[youngnick]

LGTM, thanks