Replace Arc with SharedCredentialsProvider in credentials_provider builder methods (#2227)

* Stop exposing `Arc` in `credentials_provider` builder methods

This commit rectifies the signature changes made to `credentials_provider`
builder methods in https://github.com/awslabs/smithy-rs/pull/2122

.
The PR in question removed `SharedCredentialsProvider` and the builder
methods started taking `ProvideCredentials` in `Arc` instead. This causes
a problem for us to evolve the API in the future as the API forces the user
to be aware of the implementation detail that we use `Arc`.

* Update comments for `SharedCredentialsCache`

* Fix unresolved doc links to `ProvideCredentials`

This commit fixes unresolved doc links to `ProvideCredentials` in
`aws_credential_types::lazy_caching`.

* Update CHANGELOG.next.toml

Co-authored-by: Yuki Saito <awsaito@amazon.com>