feat: Support basePath in options-per-service for openapi generator - Option 2

[KavithaSiva]

Closes SAP/cloud-sdk-backlog#1259.

This approach enables customising the basePath both before and after client generation.

It changes the generated client code to also add an additional basePath and pass it on to the OpenApiRequestBuilder.

export const CollectionsApi = {
  _defaultBasePath: '/base/path/to/service',
  
  createCollection: (body: CollectionRequest) =>
    new OpenApiRequestBuilder<any>(
      'post',
      '/collections',
      {
        body
      },
      // Forced to use the fully qualified name, as `this._default_basePath` results in Object is possibly 'undefined'.
      CollectionsApi._defaultBasePath
    )
};

A new setBasePath() method is also introduced in the OpenApiRequestBuilder.
Usage from a user would look like this:

await CollectionsApi.CreateCollection({
  title,
  embeddingConfig: {},
}).setBasePath(`/some/base/path`).execute(destination);

• I know which base branch I chose for this PR, as the default branch is v3-main now, which is not for v4 development.
• If my change will be merged into the main branch (for v4), I've updated (V4-Upgrade-Guide.md)[./V4-Upgrade-Guide.md] in case my change has any implications for users updating to SDK v4

I have addressed all review comments.

[ZhongpinWang]

@tomfrenken Fyi since it relates to some of your code changes like two years ago. Here without a leading slash is the same behaviour as in OData, where remove (leading/trailing) slashes functions were used at almost each and every step to ensure no slashes at all. But it feels kind inconsistent to me because without setting the base path, we use and probably also recommend adding a leading slash at the path.

[ZhongpinWang]

Overall LGTM. Thanks for this nice contribution.

Be sure to add this to v4.