feat(plugin-seo): improve getCover

[pengzhanbo]

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Provide a description in this PR that addresses what the PR is solving. If this PR is going to solve an existing issue, please reference the issue (e.g. close #123).

What is the purpose of this pull request?

• Bug fix
• New feature
• Other

Description

Currently, the plugin-seo assumes that the cover image configuration field is a string, but the actual situation may be more complex. For example, if cover is of an object type, the current getCover may encounter errors that lead to build failures.

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[coveralls]

Pull Request Test Coverage Report for Build 12353801220

Details

• 5 of 12 (41.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 60.167%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
plugins/seo/plugin-seo/src/node/utils/getCover.ts	5	12	41.67%

Totals
Change from base Build 12353692663:	-0.03%
Covered Lines:	1311
Relevant Lines:	1975

---

💛 - Coveralls

[Mister-Hope]

Can you explain why object format of cover and banner are used?

We can provide a set of getters like the feed plugin, that sounds more natural to me.

[pengzhanbo]

For example, in the theme plume, you can configure the cover image in more detail.

[pengzhanbo]

Implementing the getter in this plugin is another task that is not within the scope of this PR; this PR is solely an improvement to the default getCover.

[Mister-Hope]

But the main problem is that I think assuming cover is string is fine. The plugin is designed to be out of box.

If you want to provide further information about the cover, setting it with coverSettings or coverStyle should be fine, so I would rather accept a PR supporting getter directly.