Apply limits to the CosmosDB Key Length

[cleemullins]

Fixes #1197

This code updates the CosmosDB key scrubbing class to enforce the 255 character length defiened by CosmosDB.

[coveralls]

Pull Request Test Coverage Report for Build 48747

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.05%) to 72.21%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Builder.Azure/CosmosDBKeyEscape.cs	4	89.74%

Totals
Change from base Build 48695:	0.05%
Covered Lines:	3747
Relevant Lines:	5189

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💛 - Coveralls

[benbrown]

Here is the Javascript port of this fix: microsoft/botbuilder-js#760

[EricDahlvang]

Tested with Teams with ConversationData and PrivateConversationData ... lgtm!