Added x-forwarded-client-cert server interceptor

[rmichela]

XfccServerInterceptor parses the x-forwarded-client-cert (XFCC) header populated by TLS-terminating reverse proxies. For example, Istio and Linkerd. If present, the parsed XFCC header is appended to the gRPC Context.

• Envoy XFCC Header
• Linkerd XFCC Header

[codecov]

Codecov Report

Merging #64 into master will increase coverage by 1.19%.
The diff coverage is 88.88%.

@@             Coverage Diff              @@
##             master      #64      +/-   ##
============================================
+ Coverage     69.31%   70.51%   +1.19%     
- Complexity      176      181       +5     
============================================
  Files            30       31       +1     
  Lines           691      736      +45     
  Branches         61       67       +6     
============================================
+ Hits            479      519      +40     
- Misses          196      200       +4     
- Partials         16       17       +1

Impacted Files	Coverage Δ	Complexity Δ
...rpc/contrib/interceptor/XfccServerInterceptor.java	88.88% <88.88%> (ø)	5 <5> (?)

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[codecov]

Codecov Report

Merging #64 into master will increase coverage by 2.54%.
The diff coverage is 89.9%.

@@             Coverage Diff              @@
##             master      #64      +/-   ##
============================================
+ Coverage     69.09%   71.63%   +2.54%     
- Complexity      193      237      +44     
============================================
  Files            33       37       +4     
  Lines           783      892     +109     
  Branches         65       90      +25     
============================================
+ Hits            541      639      +98     
- Misses          224      228       +4     
- Partials         18       25       +7

Impacted Files	Coverage Δ	Complexity Δ
...m/salesforce/grpc/contrib/xfcc/XfccMarshaller.java	100% <100%> (ø)	3 <3> (?)
...force/grpc/contrib/xfcc/XfccServerInterceptor.java	81.81% <81.81%> (ø)	4 <4> (?)
...sforce/grpc/contrib/xfcc/XForwardedClientCert.java	82.05% <82.05%> (ø)	16 <16> (?)
...a/com/salesforce/grpc/contrib/xfcc/XfccParser.java	96.42% <96.42%> (ø)	21 <21> (?)

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