[TraceGen] Add support to pass in custom headers for the oltp calls made via tracegen

[pavankrish123]

Tracegen is very invaluable in our load testing. It would be even helpful if we can pass in test/basic-auth headers along with http/grpc export calls. Otel Go SDK already supports an option to add headers to export calls, so it is a matter of reading map[string]string via tracegen command line as config and passing them on

something like will do.

	if len(cfg.Headers) > 0 {
		grpcExpOpt = append(grpcExpOpt, otlptracegrpc.WithHeaders(cfg.Headers))
		httpExpOpt = append(httpExpOpt, otlptracehttp.WithHeaders(cfg.Headers))
	}

[pavankrish123]

Please assign this to me. Issued a PR #7309 already