feat: add omitempty prefix to rule path

[tonybase]

flags:
-omitempty=false
-omitempty_prefix=/api

protoc --proto_path=.  --go-http_out=omitempty=false,omitempty_prefix=/api,paths=source_relative:. service.proto

rpc SayHello (HelloRequest) returns (HelloReply) {
    option (google.api.http) = {
        get: ""
    }
}

generated path:
/api/helloworld.v1.Greeter/SayHello

[codecov-commenter]

Codecov Report

Merging #2870 (e938bd9) into main (e86ad24) will increase coverage by 0.05%.
The diff coverage is n/a.

❗ Current head e938bd9 differs from pull request most recent head e8bce5b. Consider uploading reports for the commit e8bce5b to get more accurate results

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@@            Coverage Diff             @@
##             main    #2870      +/-   ##
==========================================
+ Coverage   84.42%   84.47%   +0.05%     
==========================================
  Files          88       88              
  Lines        3936     3936              
==========================================
+ Hits         3323     3325       +2     
+ Misses        437      435       -2     
  Partials      176      176              

see 1 file with indirect coverage changes