Define a default text style to the Form Row labelAppend prop

**Requestor**

@joana-cps

**Problem Statement**

The `EuiFormRow` `labelAppend` prop is type any and by default renders a big text that very often needs to be reduced by different solutions manually.

**Proposed Solution**

Provide a more appropriate default style to the text passed to this prop but keep letting pass a custom React element if needed.

Form `16px` and `black` -> `xs` and a `subdued`

**Use Case**

**Value / Impact**

- Brings more consistency
- Removes overhead on tweaking the output manually by different solution teams

**Urgency**

**Do alternatives or workarounds exist?**

**Related code or customizations**

**Additional context (Optional)**

**Designs or Specs (Optional)**

Start by locating the EuiFormRow implementation and its labelAppend prop handling, then inspect nearby component tests or stories. Verify how plain text and custom React elements are rendered, and confirm that plain text uses the requested xs and subdued styling while custom elements remain supported.

Written by the indexing model from the issue text.