feat: adds reserved name support for PHP and reserved name tests

[bshaffer]

A new API coming out soon has a proto class Namespace, which is breaking for PHP client libraries.

See https://github.com/protocolbuffers/protobuf/pull/7268/files, which ensures the protobuf c library follows the same rules

[codecov]

Codecov Report

Merging #3100 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master   #3100      +/-   ##
===========================================
- Coverage     87.14%   87.1%   -0.05%     
+ Complexity     6045    6038       -7     
===========================================
  Files           494     494              
  Lines         23955   23954       -1     
  Branches       2599    2599              
===========================================
- Hits          20876   20864      -12     
- Misses         2225    2230       +5     
- Partials        854     860       +6

Impacted Files	Coverage Δ	Complexity Δ
.../com/google/api/codegen/util/php/PhpTypeTable.java	84.21% <100%> (+3.56%)	16 <6> (+3)	⬆️
...ransformer/ruby/RubyApiMethodParamTransformer.java	92.59% <0%> (-6.18%)	24% <0%> (-3%)
...ransformer/py/PythonApiMethodParamTransformer.java	94.33% <0%> (-4.72%)	24% <0%> (-3%)
...api/codegen/transformer/ruby/RubySurfaceNamer.java	93.46% <0%> (-1.01%)	92% <0%> (-1%)
...om/google/api/codegen/config/FlatteningConfig.java	72.34% <0%> (-0.86%)	49% <0%> (-2%)
.../google/api/codegen/config/GapicProductConfig.java	82.24% <0%> (-0.04%)	105% <0%> (ø)
...e/api/codegen/transformer/InitCodeTransformer.java	95.31% <0%> (+0.17%)	118% <0%> (-1%)	⬇️

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[yihanzhen]

LGTM