Write guidelines in Azure/azure-sdk

[heaths]

See https://github.com/Azure/azure-sdk-for-rust/issues?q=is:issue%20label:guidelines for a list of issues to consider.

Things to consider:

• [x] Always return azure_core::Result from client methods.
• [x] When to use impl Into<String> vs. impl AsRef<str>. Are there clear guidelines for when to use Cow<'_, str>?
• [x] We should get rid of preludes. See internal discussion for context. Basically, too much ambiguity and risk of name collisions.
• [x] Pageables should be reconstitutable. Does this mean, like Go, we have separate iterators for each page and each item within each page? I think we can support both that and simple iterators like .NET's enumerables. Rust is low-level but chalk full of helpful higher-level abstractions.
  • Comparing with AWS at https://docs.aws.amazon.com/sdk-for-rust/latest/dg/paginating.html, I think returning an IntoIterator<T> makes much more sense and is more idiomatic. We could still achieve reconstitutable pageables by taking in a nextLink (much like taking a status monitor URL for an LRO) and pick up where you left off. You can use any of the many Iterator methods to skip, take, collect, etc.
• [x] LROs should be reconstitutable. This is being driven across languages.
• [x] When we allow macros and why they are safe in Rust (provide link)
• [x] Defining cargo lints and inheriting from workspace for lints, dependencies, etc.
• [x] Builders can still be used for special purposes like SAS URL builders.
• [ ] Models should go into models module keeping the client and convenience helpers in the crate root.
• [ ] Convenience layers need to be able to merge with generated source. We can look to the existing source for some ideas here.

[cataggar]

Still a good idea for the future.