Add bearertokenauth extension from contrib

[chrroberts-pure]

HI!

This PR is to add the bearertokenauth extension from contrib.

This is required as there will be a follow up PR to add the purefa receiver to the this OTel distribution, in which the purefa receiver requires this extension.

Pure Storage is a Splunk Partner with mutual customers who are looking to get their Pure Storage Data into Splunk IM.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[chrroberts-pure]

I have read the CLA Document and I hereby sign the CLA

[pjanotti]

hi @chrroberts-pure , could you please sign the commits? I will merge after that.

[chrroberts-pure]

It will be significantly easier to abandon this PR and create a new one since it's had a merge commit.