Introduce longterm environment to TSI resource hierarchy

[sandshadow]

Add a new L1 Sku for long-term environments. Introduces a new "kind" property on Environment that is used as a discriminator to determine whether the environment is "Standard" or "LongTerm". This is basically the same pattern as we use when creating event sources.

Changes are on a new preview api version. This api is initially hidden behind a feature flag for private preview of long-term environments.

Review by ARM team is being added on the pull-request into the TimeSeriesInsights fork so we can get signoff early rather than wait until it goes to master.

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[sandshadow]

@ravbhatnagar
Can you review this change to the TSI swagger spec? It's a PR into the team's fork of the repository so we can get the ARM team's review before we are ready to expose publicly.

[yeskarthik]

Also determines the "storage type" now?

[sandshadow]

I updated the description to indicate the Sku determines the type of environment.

[sandshadow]

Got email signoff from @ravbhatnagar