Allow configuration of cache TTL via properties/environment

Hi,

Very cool library. Would be curious if you would accept a PR for the following:

IMO, would be very useful to allow overriding the `CacheItemTTL` via properties or environment—similar to what is done for overriding the VPC region/URL.

For Spring applications, devs can start using your library w/ just 2 lines changed in application.yaml under `spring.datasource`. That's really powerful. But if I ALSO want to configure the cache TTL, it requires a completely in-code solution, with duplication of the default code/config relating to datasource registration and connection pooling. That's a much higher barrier to entry, and also comes with some risk and a maintenance burden.

Of all the properties w/in [SecretCacheConfiguration](https://docs.aws.amazon.com/secretsmanager/latest/userguide/retrieving-secrets_cache-java-ref_SecretCacheConfiguration.html), the cache TTL is the most likely to be overridden. The default is only 1 hr, but likely that most DB passwords are being changed on the order of months. Each Secrets Manager fetch comes with a cost, so reducing unnecessary fetches has a financial incentive at scale—not just the desire to reduce CPU cycles and network load.

Also, this issue has come up or been mentioned a few times in the past, see #56 and #168.

Again, happy to provide a PR for this! Thanks for your time.

Start by tracing how SecretCacheConfiguration is created and how the VPC region/URL are overridden from properties or the environment. Check the Spring application.yaml usage described in the issue and review #56 and #168 for prior context. Done means CacheItemTTL can be overridden without duplicating datasource registration and connection-pooling configuration, with coverage for the supported configuration paths.

Written by the indexing model from the issue text.