[shared storage] Introduce SharedStorageModifierMethod union Mojo type

What:
- Introduce SharedStorageSet(/Append/Delete/Clear)Method and a union
  type SharedStorageModifierMethod. Consolidate the existing modifier
  methods into a single
  `SharedStorageUpdate(SharedStorageModifierMethod)` method.
- No behavior change.

Why:
- Enable Batch Updates: This prepares for the upcoming support for the
  JavaScript API, batchUpdate(sequence<SharedStorageModifierMethod>),
  allowing websites to perform multiple modifications in a single
  operation. Explainer: https://github.com/WICG/shared-storage/pull/199
- Improved Maintainability: This simplifies the Mojo interface and
  reduces future maintenance overhead.

Bug: 373899210
Change-Id: I3b516b577fb170cca19bd7f9afa1c019f53e0f45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5949646
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1372139}