skus sdk native-client - add body to httpclient request for skus (uplift to 1.46.x)

[brave-builds]

Uplift of #15785
Resolves brave/brave-browser#26523

Pre-approval checklist:

• You have tested your change on Nightly.
• This contains text which needs to be translated.
  • There are more than 7 days before the release.
  • I've notified folks in #l10n on Slack that translations are needed.
• The PR milestones match the branch they are landing to.

Pre-merge checklist:

• You have checked CI and the builds, lint, and tests all pass or are not related to your PR.

Post-merge checklist:

• The associated issue milestone is set to the smallest version that the changes is landed on.

[stephendonner]

Verified PASSED using

Brave	1.47.56 Chromium: 107.0.5304.91 (Official Build) nightly (x86_64)
Revision	3d5948960d62418160796d5831a4d2d7d6c90fa8-refs/branch-heads/5304@{#1097}
OS	macOS Version 11.7.1 (Build 20G918)

Steps:

1. launched Brave with --vmodule="*/skus/*"=1 --enable-logging=stderr --v=0
2. opened brave://flags
3. set brave://flags/#brave-vpn to Enabled
4. set brave://flags/#skus-sdk to Enabled (not necessary, as it's now Enabled by default, force of habit)
5. relaunched Brave (with commandline intact)
6. loaded account.bravesoftware.com
7. entered username/password from 1PW
8. used a throwaway email address from mailinator.com - [email protected]
9. clicked Get login link button
10. clicked on the link in the email I got
11. clicked Browse plans
12. scrolled down
13. clicked Buy now for Brave VPN Subscription
14. completed purchase through Stripe (https://stripe.com/docs/testing#testing-interactively)

Confirmed I was able to purchase, get issued credentials for, and use the Brave VPN connection 👍

[kjozwiak]

Uplift into 1.46.x approved 👍 QA has verified the PR on Nightly as per #15833 (comment).