[FIX] PIA receive token API - use form data instead of JSON

[Kiougar]

Breaking change

Proposed change

The API call to receive the token from PIA stopped working (received a 500 response from the server).

Most likely PIA changed the API on their side and POSTing with JSON data is no longer accepted.

Changing it to a POST with form data solves the issue.

PIA does the same in their repo:
https://github.com/pia-foss/manual-connections/commit/9b42ad934a9353fbefda7cbab2725f4f3b1850a3#diff-7bfb1a856c312babfe1d7153f2b8ee316404fa832e6230a77f5d290dd1ad4486

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New provider (thank you!)
• Updated provider (thank you!)
• New feature (which adds functionality to a provider script/repo usage)
• Breaking change (fix/feature causing existing functionality to break)

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue: relates to #
• Link to documentation updated (if done separately): https://...

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated