fix(iam): Decode policy document before parsing

[MisterMX]

Description of your changes

Fixes a bug that was introduced in #1774 which causes reconcile to fail during parsing of an URL-encoded policy document.

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

Unit tests and the existing examples.

[github-actions]

Backport failed for release-0.40, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git fetch origin release-0.40
git worktree add -d .worktree/backport-1789-to-release-0.40 origin/release-0.40
cd .worktree/backport-1789-to-release-0.40
git checkout -b backport-1789-to-release-0.40
ancref=$(git merge-base 632548323529128edbc13e7be35e19cb1dab58f9 58341a021e5c4e826819886abc2a31e3242d3169)
git cherry-pick -x $ancref..58341a021e5c4e826819886abc2a31e3242d3169