Adapt to external URI that might not be a URL

[JohnNiang]

The external URI might not be an URL, so it would not work well. See https://github.com/halo-dev/halo/issues/3762 for more.

This PR

• adapts to external URI that might not be a URL
• refactors cache key with SitemapGeneratorOptions to make sure the different request URLs have different keys
• upgrades to Gradle 8.0.2
• refactors build script

BTW, I have tested this plugin on Halo 2.0-2.4.

You could test this plugin manually using plugin-sitemap.zip.

/kind bug

修复 External URL 为“/”时无法访问的问题

[JohnNiang]

/cc @halo-sigs/halo

[f2c-ci-robot[bot]]

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[JohnNiang]

/ping @halo-sigs/halo

[f2c-ci-robot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ruibaby

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[JohnNiang]

I'm going to merge this PR manually, because the automatic merge is unavailable currently.