feat(storage): Set CRC32C as the default checksum option

[salilg-eng]

This PR introduces a change to the insertObject method in the Storage client to set CRC32C as the default checksum for all object uploads.

Why this change is needed
Previously, if a user didn't explicitly specify a validation option, no checksum was calculated or sent to the server. By defaulting to CRC32C, we can ensure a higher level of data integrity during uploads without requiring an extra configuration step from the user. This aligns the library with best practices for interacting with Google Cloud Storage and provides a better out-of-the-box experience.

Implementation details

• The resolveUploadOptions method was updated to check for the presence of the validate key. If it's missing and the user hasn't provided their own checksum in the metadata, validate is now set to crc32c.
• A new test case was added to verify that the library correctly falls back to the default CRC32C validation when no other options are provided.
• An "Undefined array key" error in chooseValidationMethod was also fixed to ensure it handles cases where the validate key is not present.

This change is non-breaking and improves the robustness of the client library.

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[bshaffer]

This seems to be a duplicate of #8539, which was closed due to concerns regarding backwards compatibility, as noted in #8539 (review).

I would suggest not making this change until those concerns are addressed, or you can release a new major version of Storage (e.g. v2.0.0) to protect existing users from breaking when they update their google/cloud-storage dependency (I can help with the new major version release, if that's what you choose to do).

At the end of the day, the Storage team maintains this library, not Cloud SDK (my team), so you don't require my approval to merge this change. This is simply my recommendation, and I will withhold approval until the concerns have been addressed.