[upstream #4965] feat(core): Auto-initialise customFields for entities that support them

[ayim]

Mirrored from vendurehq#4965 for the Overwatch review demo.

Original author: @grolmus

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Summary

Auto-initialises an empty customFields array for every entity that supports custom fields, so a plugin's configuration callback can extend any such entity (core or plugin-defined) without the defensive guard:

// Before — guard required, or startup crashes with "Cannot read properties of undefined"
configuration: config => {
    if (!config.customFields.CompanyRole) (config.customFields as any).CompanyRole = [];
    config.customFields.CompanyRole.push({ name: 'spendingLimit', type: 'int' });
}

// After — just works
configuration: config => {
    config.customFields.CompanyRole.push({ name: 'spendingLimit', type: 'int' });
}

Resolves OSS-408.

How

• New getEntityNamesWithCustomFields() (register-custom-entity-fields.ts) detects custom-field-capable entities from the TypeORM metadata (they declare a customFields embedded column) — a HasCustomFields implements check isn't available at runtime.
• runPluginConfigurations() (bootstrap.ts) pre-seeds config.customFields[EntityName] = [] for each of those before the plugin configuration callbacks run, only where an entry doesn't already exist (never overwrites configured fields).

Empty arrays are ignored by registerCustomEntityFields (it already skips zero-length configs), so this is inert for entities nobody extends — no schema/column changes.

The TypeScript half of the original issue (the as any footgun) is already resolved on main: CustomFields now carries a & { [entity: string]: CustomFieldConfig[] } index signature, so config.customFields.CompanyRole is typed without a cast. This PR adds the matching runtime guarantee.

Tests

bootstrap.spec.ts — custom-field-capable entities get [] initialised; existing entries are preserved; and a @VendurePlugin({ configuration }) can push to another entity's custom fields without a guard. Source typechecks clean; related entity specs pass.

Relates to OSS-408

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