refactor: tracing injection mechanism

[dmehala]

This commit introduces a refined header injection mechanism that modify the request structure with headers to inject, instead of mutating nginx's configuration with proxy_set_header, fastcgi_param, grpc_set_header or uwsgi_param.

Resolves #19 Resolves https://github.com/DataDog/nginx-datadog/issues/43

Changes:

• Add a DictWriter implementation for nginx.
• Add a test covering issue #19.
• Deprecate opentracing_*_propagate_context.
• Remove tests no longer necessary.
• Update existing tests to align with the new implementation.