support search indexing and searching for multi-instance fields

[i-just]

Description

We can no longer rely solely on the one-to-one relationship between the field handle and fieldId.

• Added a migration that adds the layoutElementUid column to the searchindex table.
• Updated primary key to factor in that new column.
• Updated the indexing logic to store the layoutElementUid.
• Updated the search logic to rely primarily on the layoutElementUid and fall back on the fieldId.

Upgrade impact:
When updating from c4 to c5, after all migrations are run, a resave command with update search index param should be run for all element types that have custom field layouts (e.g. craft resave/entries --update-search-index=1, craft resave/addresses --update-search-index=1 etc.). This is so that layoutElementId is populated in the searchindex table. This is needed to support the indexing and searching through the multi-instance fields.

Related issues

#13987