SecretsManagerTemplate use cases and design

[arunkpatra]

The concept of a SecretsManagerTemplate might be useful for clients in many ways, primary among which are:

• Allow users access to an underlying SecretsManagerClient with useful lifecycle operations like connect and close operations baked in, thereby alleviating the user from including boilerplate in their code.
• Allow users to do whatever operation they want to do against AWS Secrets Manager service via the SecretsManagerTemplate

These kind of facilities are beyond the standard needs that Spring Cloud AWS Secrets Manager module supports today, which is to source config props and have them injected into the Environment.

@eddumelendez @MatejNedic

EDIT: We still could need to decide if we want to keep the underlying connection open since establishing a connection is expensive latency wise.