Provide ability for clients to add HTTP headers

[albertteoh]

Which problem is this PR solving?

Resolves: #478

When services, such as jaeger-collector, are deployed to production, they are assigned ephemeral IP addresses and ports when listening for HTTP requests.

In order for caller services (whether calling from development or production) to reach specific endpoints of callee services, a side-car process is used to route the outbound request to the correct IP and port which is achieved by providing the:

• caller service name
• target service name
• target HTTP endpoint

Currently, jaeger-client-go does not provide a capability for clients to flexibly add headers to the outbound HTTP request.

Short description of the changes

• Add a key-value map Headers to ReporterConfig
• This allows clients to configure custom headers which will be passed to the NewHTTPTransport
• These custom headers will be set in the outbound HTTP request when submitting spans

[codecov]

Codecov Report

Merging #479 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #479     +/-   ##
=========================================
+ Coverage    88.1%   88.21%   +0.1%     
=========================================
  Files          59       59             
  Lines        3507     3512      +5     
=========================================
+ Hits         3090     3098      +8     
+ Misses        305      302      -3     
  Partials      112      112

Impacted Files	Coverage Δ
transport/http.go	83.82% <100%> (+1.28%)	⬆️
config/config.go	95.3% <100%> (+2.01%)	⬆️

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