Protect trace API token by passing it through an environment variable

[NipunaMadhushan]

Purpose

Currently, users have to pass the trace api token in the reporterEndpoint configuration and it may lead to security issues. This will provide the user the capability of passing the trace API token with an environment variable named as TRACE_API_TOKEN. Therefore users do not need to pass the trace api token with trace api URL anymore.

Goals

Reduce security issues while using zipkin library.

Fixes #5

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (00f33b4) 0.00% compared to head (bf3762c) 0.00%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main      #6   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          3       3           
  Lines         56      60    +4     
  Branches       5       6    +1     
=====================================
- Misses        56      60    +4     

Files	Coverage Δ
...ina/observe/trace/zipkin/ZipkinTracerProvider.java	0.00% <0.00%> (ø)

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