Add decision log for SRA Identity changes (#3905)

Author: gosar

docs/design/core/sra-identity-auth/DecisionLog.md

@@ -0,0 +1,49 @@
+# Decision Log for Smithy Reference Architecture Identity and Auth support 
+
+## Log Entry Template
+
+**Source:** (Meeting/aside/pair programming discussion/daily standup) to (discuss/implement) X
+
+**Attendees:** (names)
+
+**Closed Decisions:**
+
+1. Question? Decision. Justification.
+
+**Open Decisions:**
+
+1. (Old/Reopened/New) Question?
+
+## 3/31/23
+
+**Source:** Meeting for API surface area review of Identity changes made as part of SRA.
+
+**Attendees:** Anna-Karin, David, Debora, Dongie, Jay, John, Matt, Olivier, Zoe
+
+**Closed Decisions:**
+
+1. **Should the new interface `AwsCredentialsIdentity` provide `create()` methods?**
+   1. Yes, to provide customers a way to easily create instances of this type without needing to depend on 
+      `AwsBasicCredentials` from the `auth` module. Some duplication of code from `AwsBasicCredentials` is okay.
+   2. The implementation of `create()` can use an anonymous inner class, instead of creating a new class with a name.
+2. **How should `AwsCredentialsProviderChain` support the new Identity type `AwsCredentialsIdentity`?**
+   1. `Builder.addCredentialsProvider()` can be overloaded to accept the new type.
+   2. The varargs methods `of()` and `Builder.credentialsProviders()` can be overloaded to accept the new type. This
+      would not be ambiguous when called with zero args, because according to
+      https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.12.2.5 the more specific method is chosen, 
+      i.e., `AwsCredentialsProviderChain.of()` would invoke `of(AwsCredentialsProvider...)`.
+   3. The `Builder.credentialsProviders()` method accepting a `Collection` cannot be overloaded because both methods
+      would have the same erasure. So use a method with a different name - `credentialsIdentityProviders()`. We don't
+      want to add `Identity` to the other methods (varargs, `add`, `of`) too, as it might mislead to thinking that's a 
+      different "property" of the chain. So a separate method name is used only in this one-off.
+3. **How should an `IdentityResolver` define which `IdentityProperty`s it supports?**
+   1. An `IdentityResolver` should define a `public static` for each `IdentityProperty` it supports and document the
+      behavior of how it uses it during `resolveIdentity`. This will help the caller determine how to construct an
+      appropriate `ResolveIdentityRequest`. 
+   2. Should there be stronger abstraction for any property?
+      1. We discussed potential use cases like metrics collector / telemetry. If we have a compelling use case, we can
+         add it later. Though it would have to be not AWS specific to be added to these generic interfaces.
+
+**Open Decisions:**
+
+None