Add multiple cdn support

[peng]

Changes

This is not final PR. If my code is ok, later I will write test and docs.

• What does this change?

Add build.assetsPrefix is also function.

Astro not support multiple cdn. We disscuss is this issues: #9700 (comment)

I have opened a discussion here: withastro/roadmap#818

Testing

Docs

If my idea and code is ok, later I will write test and docs. If not, please tell me how to modity it. Thanks!

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[ematipico]

This approach, unfortunately, doesn't work in SSR. My suggestion is to have an object like this:

{
	"default": "https://cdn.example.com",
	"js": "https://cdn-js.example.com",
}

The new logic will match the file extension with the key of the object. If nothing is in there, you can use the default.

[peng]

This approach, unfortunately, doesn't work in SSR. My suggestion is to have an object like this:

{
	"default": "https://cdn.example.com",
	"js": "https://cdn-js.example.com",
}

The new logic will match the file extension with the key of the object. If nothing is in there, you can use the default.

OK, I will solve it.