Allow support for custom headers

[FerPerales]

It may be useful to allow a custom_headers option, similar to Snowplow JS library, to send arbitrary headers

[snowplowcla]

Thanks for your pull request. Is this your first contribution to a Snowplow open source project? Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://docs.snowplowanalytics.com/docs/contributing/contributor-license-agreement/ to learn more and sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

[FerPerales]

Thanks for your pull request. Is this your first contribution to a Snowplow open source project? Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://docs.snowplowanalytics.com/docs/contributing/contributor-license-agreement/ to learn more and sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

Signed it

[snowplowcla]

Confirmed! @FerPerales has signed the Contributor License Agreement. Thanks so much.