Adding environment variables for Http headers

[ashu658]

Changes

Add environment variables to be configured for capturing http headers.
This spec describes how to capture the http request/response header and add them in the span. It does not specify the environment variables need to be configured to capture the headers. This change specifies these environment variables.

Env vars proposed in the PR are already supported in Java and Python sdks.

Related PR: open-telemetry#1898

Additional info:
One thing that I have seen in python is that headers provided by some frameworks/libraries (e.g. wsgi based frameworks like flask) are lowercase and - is replaced with _ which would mean user who configures
OTEL_INSTRUMENTATION_HTTP_CAPTURE_HEADERS_SERVER_REQUEST="CusTOM_HEAder-1"
will be able to capture custom-header-1 from request headers for some frameworks/libraries whereas for some other frameworks he won't be able to capture this header as it does not change - to _.
(All python frameworks/libraries I saw provides case-insensitive headers but only some of them replace - with _)
Mentioning this info here as I am not sure if it might affect this spec or not.

[sanketmehta28]

Add a note in markdown that actual header will be normalized once transformed to span attributes. i.e. "Custom-Header-1" will be transformed to http.request.header.custom_header_1.