amp-autocomplete: Please add XSSI prefix support

### Description

Unlike every other extension component I've used to date, ```amp-autocomplete``` does not support the AMP-standard XSSI prefix stripping for remote data sources.

This would obviously be useful, alongside the CORS specification, in stopping XSSI and similar (data-scraping) attacks.

### Alternatives Considered

One could create a distinct CORS endpoint that does not return the XSSI prefix, but that only solves the superficial issue of getting the component working; it still leaves it open to exploitation.

### Additional Context

_No response_

Start by locating amp-autocomplete's remote data-source handling and compare it with XSSI prefix stripping in the other extension components mentioned by the issue. Done means amp-autocomplete accepts AMP-standard XSSI-prefixed responses without requiring a separate CORS endpoint, with tests covering the behavior.

Written by the indexing model from the issue text.