Fix handling of relative STAC links

[uclaros]

Description

Resolve relative links to STAC Catalogs, Collections and Items into absolute links.
The link with self relation is used for resolving.

QgsStacParser now has a setBaseUrl() method to set the url which will be used to resolved any relative links (both QgsStacLink and QgsStacAsset hrefs)

[github-actions]

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit 9628a43)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit 9628a43)

[qgis-bot]

The backport to release-3_40 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 5d8633b34f3... Resolve relative STAC links
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config advice.mergeConflict false"

stdout

Auto-merging src/core/stac/qgsstaccontroller.cpp
Auto-merging src/core/stac/qgsstacparser.cpp
CONFLICT (content): Merge conflict in src/core/stac/qgsstacparser.cpp
Auto-merging src/core/stac/qgsstacparser.h
Auto-merging tests/src/core/testqgsstac.cpp

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-3_40 release-3_40
# Navigate to the new working tree
cd .worktrees/backport-release-3_40
# Create a new branch
git switch --create backport-59199-to-release-3_40
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 5d8633b34f309f18e99c65b2935414934e1eeb4f,be864209e9ef2fb36bccafaa2fddab2693c06d7e,9628a43d324c83bbc90b1ca3f5a0eb1c0b0d415f
# Push it to GitHub
git push --set-upstream origin backport-59199-to-release-3_40
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_40

Then, create a pull request where the base branch is release-3_40 and the compare/head branch is backport-59199-to-release-3_40.