chore: Remove dependency on AWSSDKIdentity unless necessary

[sichanyoo]

Companion PR: smithy-lang/smithy-swift#789

Issue #

Description of changes

• Use resolver chain for bearer token only if service actually uses HTTP bearer auth. For services that do not use bearer auth, use static token resolver instead, which is part of SmithyIdentity module.
• This allows removal of unnecessary dependency on AWSSDKIdentity.
• Also, make DefaultBearerTokenIdentityResolverChain be imported as struct

New/existing dependencies impact assessment, if applicable

Conventional Commits

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jbelkins]

When I code generate with this branch, every service still has a AWSSDKIdentity.DefaultBearerTokenIdentityResolverChain. Do you see the same?

(also, a nit: modify the DefaultBearerTokenIdentityResolverChain symbol so that it imports as a struct.)

[jbelkins]

Pending green build. Re-perform codegen in separate PR after merge.