Allow custom http.Client to be passed to SR client

[rayokota]

Allow custom http.Client to be passed to SR client

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ rayokota
❌ ConfluentJenkins
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ rayokota
❌ ConfluentJenkins
_{You have signed the CLA already but the status is still pending? Let us recheck it.}