Proposal to rename refresh APIs  #350

Due to some confusion around the use of the word "cache" in our API names, we could change the following APIs to better reflect their functionality:
AzureAppConfigurationRefreshOptions

• SetCacheExpiration -> SetRefreshInterval

FeatureFlagOptions

• CacheExpirationInterval property -> Deprecate the property in favor of a new SetFeatureFlagRefreshInterval method

This would help clarify that the provided TimeSpan is only used to trigger a new refresh operation. If nothing has changed on the server, configuration values being used by the application continue to be valid even after this time has passed - values don't "expire".

The proposed name changes also align with our existing AzureAppConfigurationKeyVaultOptions.SetSecretRefreshInterval API.

cc Drago Draganov (@drago-draganov) Zhenlan Wang (@zhenlan)