feat: support Elasticache cluster mode by setting IsClusterMode in config

[reeshijoshi]

• This is required because Elasticache supports setting up cluster mode with configuration endpoint. So the check len(Addrs) > 1 is not enough for determining cluster mode.
• If IsClusterMode is true, the client will be created as a cluster client.

[ndyakov]

@reeshijoshi thank you for the contribution. Would you be able to add tests for this new option?

[reeshijoshi]

@reeshijoshi thank you for the contribution. Would you be able to add tests for this new option?

This needs an Elasticache configuration endpoint to be provided in the Addrs field. I can add a unit test with one address only in this but that's not representative of how this would behave in the actual scenario

[reeshijoshi]

@reeshijoshi thank you for the contribution. Would you be able to add tests for this new option?

Added a test that asserts client type

[ndyakov]

@vladvildanov I think this one is good to be merged.

[ndyakov]

@reeshijoshi Overall looks good to me, would you mind changing the comment as suggested or anything starting with the IsClusterMode as suggested in https://tip.golang.org/doc/comment

[ndyakov]

Looks good to me, thanks @reeshijoshi

[ndyakov]

@reeshijoshi would you please add additional test so we can merge this?

[reeshijoshi]

@reeshijoshi would you please add additional test so we can merge this?

done