Added $filter param to list databases

[jaredmoo]

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[jaredmoo]

New test case is in Azure/azure-sdk-for-net#2847

[dsgouda]

LGTM. @olydis just curious, renaming the enum should not be a breaking change correct? I know it isn't for C# but not sure about others.
Also, feel free to merge

[jaredmoo]

Can I get a merge here please?

[AutorestCI]

Azure/azure-sdk-for-ruby#704

[AutorestCI]

No modification for NodeJS

[AutorestCI]

Azure/azure-sdk-for-python#1014

[jaredmoo]

Thanks!