Support for Custom `REQUEST` Methods in `TAPService` Queries

### Description:
I am proposing an enhancement to the `pyvo.dal.TAPService` functionality to support custom `REQUEST `methods, such as `doQueryFilteredByDistance`, in addition to the default `doQuery`. This feature will provide greater flexibility and utility for users, particularly in the context of integrating the `TAPService` with the `sunpy-soar` module.

### Context:
The current implementation of the `pyvo.dal.TAPService` class hardcodes the `REQUEST ` parameter to `doQuery`. However, there are scenarios where alternative `REQUEST `methods are required to perform more specialized queries, such as filtering results by distance using `doQueryFilteredByDistance`. This was identified during the development and integration efforts within the `sunpy-soar `project, where the ability to perform such queries would be helpful for effectively interfacing with the Solar Orbiter Archive (SOAR) API.

### Proposed Solution:
Enhance the `TAPService ` class to accept an ` request_type` argument, with the default value set to `doQuery`. This argument will allow users to specify the desired `REQUEST ` method when constructing their queries.

Start at pyvo.dal.TAPService and trace where TAP queries hardcode the REQUEST method to doQuery. Preserve doQuery as the default, allow request_type to select custom methods, and verify both default and custom query behavior in the relevant tests.

Written by the indexing model from the issue text.