Allow icu_testdata's BakedDataProvider to be broken down by component and experiment

[sffc]

Currently icu_testdata's BakedDataProvider is monolithic and pulls in all ICU4X components, including experimental ones. The feature is generally disabled because of the compile times. Both of these issues could be resolved if we made each part of BakedDataProvider be feature-gated. For example, "icu_datetime" could dev-depend on "icu_testdata/icu_datetime" (and whichever other components it needs, like "icu_testdata/icu_decimal").

We also want for the icu_datetime/experimental feature to be optional.

• @robertbastian

[sffc]

For crates with experimental features, here is what I propose

1. Add a testdata feature called e.g. icu_datetime_experimental (optionally remove the "icu_" prefix if you do so for the other features)
2. Tests in icu_datetime that need data hidden behind icu_testdata/icu_datetime_experimental should use the serde data instead, until Allow optional dev-dependencies rust-lang/cargo#1596 is fixed.

[robertbastian]

Fixed by #2364