Add configuration for refresh token before expiry

[jianghaolu]

Add tests later but just opening a PR for design discussion.

Fixes #6182

Sample usage:

ManagedIdentityCredential credential = new ManagedIdentityCredentialBuilder()
    .clientId("user managed identity client id")
    .tokenRefreshOffset(Duration.ofMinutes(10))
    .build();

[JonathanGiles]

Generally looks good. I would like to workshop on the name, improve the docs, and run the issue past @anuchandy as well.