[FEATURE REQ] Recommend using Boolean return type for API TokenCredentialOptionsProvider.TokenCredentialOptions#isManagedIdentityEnabled

**Is your feature request related to a problem? Please describe.**
Recommend using `Boolean` return type for method `TokenCredentialOptionsProvider.TokenCredentialOptions#isManagedIdentityEnabled`, the properties bean copying is a bit annoying because the primitive type has a default value, and you can't be sure whether it is false configured by the user or the default value. It will be a break change.

**Describe the solution you'd like**
`Boolean getManagedIdentityEnabled();`

**Describe alternatives you've considered**
N/A

**Additional context**
N/A

**Information Checklist**
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