Add generate_api_text.py script

[tjprescott]

• Adds a script to help generate API.md. This will be used as the basis for other work to generate API reviews through Github.
• Adds a script that generates the proper branch structure for PR-based API reviews.

Examples

Auto Review - #47236

PR Review - #47242

Initial (New Package) Review - #47238

[Copilot]

Pull request overview

Adds a new repo script (scripts/generate_api_text.py) to generate a package-level API.md by building a wheel, running apiview-stub-generator (apistub) to produce the token JSON, and converting that JSON to markdown via Export-APIViewMarkdown.ps1. This is repo tooling intended to streamline API review artifact generation.

Changes:

• Introduces scripts/generate_api_text.py to locate a package under sdk/*/, build a wheel, run apistub, and emit API.md into the package directory.
• Adds logic to install eng/apiview_reqs.txt and (optionally) upgrade apiview-stub-generator from the Azure SDK package index.
• Uses eng/common/scripts/Export-APIViewMarkdown.ps1 to convert the generated *_python.json token file into markdown.