[Schema Registry] Support for fetching schema using group name, schema name, and version

[m-redding]

These changes add support for getting schemas from Schema Registry using their group name, schema name, and version.

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[azure-sdk]

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Azure.Data.SchemaRegistry

[m-redding]

Resolves #30968

[JoshLove-msft]

One thing I forgot to mention - we want to add a Version property to SchemaProperties now that we have a method that takes the Version.