add dfs support

[tcz001]

#10

[tombuildsstuff]

hey @tcz001

Thanks for this PR.

Taking a look through whilst this PR looks like it's off to a good start, in this SDK we're slightly customizing the API endpoints to make them easier for consumers of the SDK to use, rather than mapping them 1:1 - for example by having multiple more granular endpoints to Create a Blob of different kinds rather than creating a generic "Create Blob" function which requires the user to determine which fields should be sent.

Ultimately this means that we're intentionally not auto-generating the code and adding acceptance tests as we go - for example in the case of a DataLakeStorage Gen2 FileSystem this means it'd be good to add a parser to read the properties back into a map[string]string, rather than making consumers of the SDK parse this themselves, since this'll contain a key-value pair of base64-encoded strings.

As such whilst I'd like to thank you for this contribution, I hope you don't mind but I'm going to open another PR which takes a slightly different approach but adds support for Data Lake Storage Gen2 FileSystems for API versions 2018-11-09 and 2018-03-28 - which should both fix #10 and mean that it's possible to unblock hashicorp/terraform-provider-azurerm#4244.

Thanks!