Put date skeletons behind a feature

[sffc]

Fixes #2324

I added two features to icu_datetime:

• "experimental" for full-featured components bag
• "experimental_skeleton_matching" for a subset required by datagen (this is also a dependency of the "dategen" feature)

[robertbastian]

Activate this feature in https://github.com/unicode-org/icu4x/blob/main/components/icu/Cargo.toml#L73

[sffc]

A thorny side-issue is that this brings up questions about whether or not "datagen" is stable for clients to use. We've gone back and forth on that topic before.

[Manishearth]

suggestion: worth noting the optional SkeletonsPattern bound in the docs of the other one ("The X bound is only needed with the experimental feature and enables blah blah")

[sffc]

As a side-effect, note how provider/datagen/tests/data/work_log.wasm got substantially smaller across this change, since it doesn't need to carry skeleton resolution logic any more. When we re-enable it, we should try to make the binary size impact as small as possible.

[sffc]

@zbraniecki The bench build is failing because the benches currently require components bag. If I do like I did in the tests where I skip over the components tests when the feature is not enabled, then the benches are going to have different results depending on what features are enabled, since enabling the experimental feature makes the benches run more examples. I could:

1. Make the affected benches require the "experimental" feature
2. Decide that it is fine for the benches to have different perf characteristics based on what features are turned on
3. Block this PR until I have time to split the benches into experimental and non-experimental versions, and then make a smaller subset require the "experimental" feature

[sffc]

I think there are two issues remaining before CI is green:

1. The issue about datetime benches discussed above
2. How do we make testdata build properly when the experimental feature is disabled? We can't sprinkle #[cfg] everywhere because this is generated code, unless we change the generating code.

[sffc]

I resolved the two issues above by:

1. Turns out there is only one bench that requires components data, "datetime/datetime_components". I simply put this one behind a #[cfg].
2. I made icu_testdata/baked depend on icu_datetime/experimental. Longer term I hope we can do Allow icu_testdata's BakedDataProvider to be broken down by component and experiment #2383.

[sffc]

Yippee! Merged.

[zbraniecki]

Turns out there is only one bench that requires components data, "datetime/datetime_components". I simply put this one behind a #[cfg].

That's what I would suggest. All good!