Prevent memory leak when overriding credentials

[rcoh]

There is a fairly common practice of late-setting credentials in a config override. However, doing this naively will cause issues because each new credential you set is going through here:

smithy-rs/aws/rust-runtime/aws-credential-types/src/provider/credentials.rs

Lines 136 to 138 in 13c8bb3

	pub fn new(provider: impl ProvideCredentials + 'static) -> Self {
	Self(Arc::new(provider), IdentityCachePartition::new())
	}

This allocates a new cache partition which will then be stored in the identity cache. Customer have to know this.

There are a couple of potential ways around this:

1. When using config_override to set new credentials, don't persist these to the cache
2. Add a new method like is_static or cacheable or directly IdentityCacheLocation to ProvideCredentials:

   smithy-rs/aws/rust-runtime/aws-credential-types/src/provider/credentials.rs

   Line 86 in 13c8bb3

   pub trait ProvideCredentials: Send + Sync + std::fmt::Debug {

   then use it in the implementation of IdentityResolver to properly set the "cache location":

   smithy-rs/aws/rust-runtime/aws-credential-types/src/provider/credentials.rs

   Lines 166 to 182 in 13c8bb3

   	impl ResolveIdentity for SharedCredentialsProvider {
   	fn resolve_identity<'a>(
   	&'a self,
   	_runtime_components: &'a RuntimeComponents,
   	_config_bag: &'a ConfigBag,
   	) -> IdentityFuture<'a> {
   	IdentityFuture::new(async move { Ok(self.provide_credentials().await?.into()) })
   	}
   	fn fallback_on_interrupt(&self) -> Option<Identity> {
   	ProvideCredentials::fallback_on_interrupt(self).map(|creds| creds.into())
   	}
   	fn cache_partition(&self) -> Option<IdentityCachePartition> {
   	Some(self.1)
   	}
   	}

Either of these cases prevents the accidental caching of these identities in the identity cache leading to a memory leak.

A final stopgap measure is probably adding a cap to the size of the IdentityCache implementation as-is — A normally functioning application should not have more than 5-10 credentials providers active at any given time.