add support for es-tags-as-fields through env

[jam01]

Which problem is this PR solving?

• Fixes lack of support for es-tags-as-fields elasticsearch configuration #104

Short description of the changes

• The elasticsearch SpanDeserializer will replace tag keys through environment variables configuration.
• For the test I'm starting and closing a spark context manually and loading spans from a file, which avoids requiring a live elasticsearch instance.