implement parsing of the CONNECT-IP request

[marten-seemann]

Part of #63. Depends on #70.

This intentionally omits requests that define URI template variables, since we don't support IP flow forwarding yet.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.19%. Comparing base (bbb071c) to head (9242a16).

Files with missing lines	Patch %	Lines
request.go	89.65%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           connect-ip-capsules      #71      +/-   ##
=======================================================
+ Coverage                83.11%   83.19%   +0.07%     
=======================================================
  Files                        5        5              
  Lines                      462      476      +14     
=======================================================
+ Hits                       384      396      +12     
- Misses                      54       55       +1     
- Partials                    24       25       +1     

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