Add DocC's source link feature support

[Kyle-Ye]

IMO, we could have it by passing customDocumentationParameters eg. "--source-service-base-url https://github.com/Kyle/TestKit/blob/main" but it will lead to both main and release/1.0 branch doc to link to my main branch's source code unless we update the .spi file manully for each release.
I think we can abstract it on SPI side. So the downstream user can just opt-in this feature more easily.

If SPI only support GitHub, maybe a single bool key is enough. Otherwize, we may consider adding 2 parms
"source-service" - pass through to docc directly
"spi-source-service-base-url" (eg. https://github.com//) - we transform it to "source-service-base-url" to docc by appending "blob/main" or "blob/release/1.0"
(checkout-path is a absolute path to local checkout and is not needed to pass in in SPI, we just handle it internally and pass it to docc)

https://www.swift.org/documentation/docc/distributing-documentation-to-other-developers#Include-Links-to-your-Projects-Sources

[Kyle-Ye]

Moved to #2794