(iam): manage user ssh public keys

Implement creation, updation and deletion of IAM user SSH public keys.

### Use Case

IAM user SSH public keys are needed for CodeCommit users to access repositories via SSH.

### Proposed Solution

As a workaround, this should be achievable for a single SSH public key with an AwsCustomResource calling UploadSSHPublicKey, UpdateSSHPublicKey and DeleteSSHPublicKey, but the current documentation is too sparse for me to figure this out alone.

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request

Start by inspecting existing IAM user support and the AwsCustomResource workaround described in the issue, then review the UploadSSHPublicKey, UpdateSSHPublicKey, and DeleteSSHPublicKey operations. Done means IAM users can create, update, and delete SSH public keys for CodeCommit access, with the relevant behavior covered by the project's existing tests.

Written by the indexing model from the issue text.