[Key Vault] Stop using the spread operator in the source files

[sadasant]

The spread operator produces inconsistent data in and out the convenience layer. We should be specific about what properties we pick to send and receive.

Also, change the variable interfaces from the public API into type structures with strict properties, so that the type system can catch this if we ever introduce the spread operator again.

[sadasant]

@ramya-rao-a I'd like to merge the full LRO refactoring first: #12630

That PR has some of the untangling required for this. For example: by separating the transformations into its own file in each client's source. After that PR, I'll be able to follow up more swiftly and close this issue with a simpler set of changes.

[sadasant]

Some notes while I'm doing this:

• I'm keeping all the spread operators in the generated folders.
• I'm keeping all the spread operators on the arrays.
• I'm keeping the request options related spread operators since the RequestOptions type allows any parameter to be sent through [key: string]: any;.
• I'm keeping the tracing spread operator since tracing is already handled as cleanly as possible, I believe.
• I'm keeping the spread operator on values of types that use RequireAtLeastOne.
• I'm keeping the spread operators used in the pollers, since they're related to request options, which we receive from the user, and poller state, which we have tight control of. But feel free to challenge this!
• I'm keeping the spread operators in the tests, since changing those won't affect the customers.
• serviceVersion is being sent to InternalPipelineOptions, but InternalPipelineOptions doesn't have that property 🤔
• loggingOptions doesn't have logPolicyOptions but we were sending this everywhere 🤔