The txt response should be cacheable [#3215023]

Problem/Motivation

The response returned by the controller are not currently cacheable.

Given how often the route may be called on more complex installations, I think it makes sense to try and leverage Drupal's caching API here to reduce the load on the backend.

Steps to reproduce

N/A

Proposed resolution

Ensure that the returned responses implement the \Drupal\Core\Cache\CacheableResponseInterface interface.
As well as provide API to ensure that the response object can be invalidated as appropriate.

Remaining tasks

~~Provide an issue fork/patch.~~

User interface changes

N/A

API changes

N/A

Issue fork adstxt-3215023

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3215023-the-txt-response changes, plain diff MR !2
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