Refactor PythonIndicator to rely on compute_next_value

[JosueNina]

Description

Adds a new default initialization path for custom Python indicators, requiring overriding compute_next_value instead of update and passing the instance to the base class, while preserving backward compatibility.

Related Issue

Closes #9085

Motivation and Context

N/A

Requires Documentation Change

Yes.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which improves implementation)
• Performance (non-breaking change which improves performance. Please add associated performance test and results)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Non-functional change (xml comments/documentation/etc)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My branch follows the naming convention bug-<issue#>-<description> or feature-<issue#>-<description>

[AlexCatarino]

Perhaps you should remove the examples using update so users don't use it. I plan on flagging it in the Docs as legacy and recommend this approach.