Support exporting custom header values in opentelemetry as span attributes

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Is your feature request related to a problem? Please describe.
There are some custom headers that we are sending to tyk api gateway such as ServiceHeader: <16characters> and similar ones. Right now when opentelemetry is enabled in tyk api gateway there are only some general headers exported to otel collector, in our case, Jaeger.

This is described here: https://tyk.io/docs/product-stack/tyk-gateway/advanced-configurations/distributed-tracing/open-telemetry/open-telemetry-overview/#common-http-span-attributes

Describe the solution you'd like
I would like to have option to enable sending custom header values as additional span attributes to otel collector instead of only some general ones. This could be done via env variable:
TYK_GW_OPENTELEMETRY_CAPTURE_HEADERS:"customheader1,customheader2"

You could follow the same way as it's implemented in python instrumentation, where you can specify which http request you want to capture:
https://opentelemetry.io/docs/languages/python/automatic/example/#capture-http-request-and-response-headers

Describe alternatives you've considered
I tried to override traceparent header with our custom headers, but the format of mentioned headers are different and can't be done this way