rfc: change default identity cache and partitioning (#3544)

## Motivation and Context

Original issue: https://github.com/smithy-lang/smithy-rs/issues/3427

See the motivation section of the RFC.

## Description

Proposes a new RFC that changes the default behavior of `SdkConfig` and
`SharedCredentialsProvider`/`SharedTokenProvider` w.r.t caching and
cache partitions.

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