Replaced BasePath usage in universe tests, acm, ad, alloydb, and apigee

[modular-magician]

This is an initial set of replacements of BasePath usage in handwritten files with the transport_tpg.BaseUrl function.

Note: String concatenation is used instead of URL joining because URL joining would escape {} characters, preventing ReplaceVars from working properly.

Options considered:

• This PR
  • Pros: compile time verification; explicit about how the base URL is calculated.
  • Cons: requires rewriting large number of handwritten files; some code duplication in that config has to be passed twice.
• Pass product to ReplaceVars and implicitly add the base URL to the beginning of the URL
  • Pros: compile time verification; doesn't require passing config twice.
  • Cons: Breaks separation of concerns; ReplaceVars would no longer just be replacing vars. There's probably some usage somewhere outside of URL building that would need to be split out. More implicit behavior.
• Automatically make ReplaceVars detect {{ProductNameBasePath}} variables, trim the BasePath suffix, look up the relevant product in the registry (with the lowercase productname), and compute its BaseUrl using the config that was also passed in.
  • Pros: ReplaceVars usage could remain unchanged - many fewer files would need to be modified. (There are about 960 uses of BasePaths in services, most of which are via ReplaceVars.)
  • Cons: ReplaceVars can't have compile time checks (this is also a problem today) and base URL calculation is implicit.
  • EDIT: Planning to do this as a compatibility shim to ease the migration off config.*BasePath fields: Handle *BasePath replacement with BaseUrl func GoogleCloudPlatform/magic-modules#17385

Release Note Template for Downstream PRs (will be copied)

See Write release notes for guidance.

Derived from GoogleCloudPlatform/magic-modules#17370