Remove direct dependencies to Azure SDKs

[alzimmermsft]

Removes dependencies on azure-core, generic-core, and related SDKs to allow for more flexibility in working with non-shipped implementation of those libraries.

[srnagar]

Do we need to remove these dependencies and duplicate a lot of code that can get outdated if the core libraries change? The generated library still needs released core libraries anyway. Since core ships to Maven Central every month and ships to dev feed almost daily, using the dependency shouldn't be a blocker on the code generator.

[alzimmermsft]

Do we need to remove these dependencies and duplicate a lot of code that can get outdated if the core libraries change? The generated library still needs released core libraries anyway. Since core ships to Maven Central every month and ships to dev feed almost daily, using the dependency shouldn't be a blocker on the code generator.

I agree this ends up duplicating code from azure-core but there are some reasons behind that to me.

• This intrinsically ties the code generator to azure-core. In recent months though, we've began working and developing generic-core which has some drastically different designs. This is pushing the generators into a spot where if these diverge too deeply we'll need to jump through some complex logic to handle that.
• Many of the concepts copied from azure-core are tied deeply into Swagger / TypeSpec outputs. To me, it'd be better for the generator to own those types and map them appropriately to azure-core / generic-core / types.
• Most the copied types don't evolve, and those that do they either happen rarely (HTTP methods) or get handled in a way that doesn't change runtime behaviors of the generated code (calls to HttpHeaderName.fromString which will return the same instance as the constant).
• Last, we're reaching a point where we're looking into reducing the shipping cadence of azure-core, and related libraries. I don't think shipping cadence one way or another is a big factor for the generator.

All that said, I'll hold this PR in an unmerged state so we can discuss this further.

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[microsoft-github-policy-service[bot]]

Hi @alzimmermsft. Thank you for your contribution. Since there hasn't been recent engagement, we're going to close this out. Feel free to respond with a comment containing "/reopen" if you'd like to continue working on these changes. Please be sure to use the command to reopen or remove the "no-recent-activity" label; otherwise, this is likely to be closed again with the next cleanup pass.