fix(storage): pass signingEndpoint to URLSigner in file.getSignedUrl

[thiyaguk09]

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Fixes #8829

[gemini-code-assist]

Code Review

This pull request adds support for passing a signingEndpoint configuration option to URLSigner when generating a signed URL, along with corresponding unit tests. The feedback suggests spreading the base configuration (SIGNED_URL_CONFIG) before applying the signingEndpoint property in the test to prevent accidental overwrites.

[rajeevpodar]

When URLSigner receives this parameter, it passes it to auth.sign() as a second argument. The issue is that the public AuthClient interface only defines one parameter (sign(blobToSign: string)).

If a developer provides a custom AuthClient implementation based on our interface, Javascript will silently drop this new signingEndpoint argument at runtime. The user will receive a signed URL generated from the default endpoint, and they'll have no idea their custom config was ignored.

Suggestion: Could we update the AuthClient interface in src/signer.ts to explicitly include the optional parameter? sign(blobToSign: string, signingEndpoint?: string): Promise;

This will ensure custom implementers are aware they need to support it.

[thiyaguk09]

I've updated the AuthClient interface in src/signer.ts, it is now ​sign(blobToSign: string, signingEndpoint?: string): Promise<string>;​

[rajeevpodar]

Verify the custom auth support code flow.