[shared storage] Add 'SharedStorageModifierMethod' interface to PA worklet

Introduce `SharedStorageModifierMethod` interface to PA worklet, that
is inherited by individual methods.

This is the proposed hierarchy, as part of the Web Locks integration
proposal (e.g., This allows the IDL parameter to be written as
sequence<SharedStorageModifierMethod>, and also allows the Blink class
to store mojom object in the base class):
- https://github.com/WICG/shared-storage/pull/199
- https://github.com/WICG/shared-storage/pull/205

Bug: 373899210
Change-Id: I2c83574bf9b87ab7c0cb9e6e4c656e742415b49f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6072666
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Reviewed-by: Cammie Smith Barnes <cammie@chromium.org>
Commit-Queue: Cammie Smith Barnes <cammie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1393929}