rbac: add method name to :path in headers

[dfawley]

RBAC header matchers should be able to work with :path headers, per A41:

For this design, headers can include :method, :authority, and :path matchers and they should match the values received on-the-wire independent of whether they are stored in Metadata or in separate APIs.

Because grpc-go removes this header from what's read on the wire to what is given to applications in the metadata, we need to add it back in like we do with :method.

RELEASE NOTES:

• rbac: fix support for :path header matchers, which would previously never successfully match.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.14%. Comparing base (e912015) to head (a2c79f1).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7965      +/-   ##
==========================================
+ Coverage   82.06%   82.14%   +0.07%     
==========================================
  Files         381      381              
  Lines       38535    38539       +4     
==========================================
+ Hits        31625    31656      +31     
+ Misses       5596     5572      -24     
+ Partials     1314     1311       -3     

Files with missing lines	Coverage Δ
internal/xds/rbac/rbac_engine.go	81.21% <100.00%> (+0.46%)	⬆️

... and 23 files with indirect coverage changes