Blob metadata is not validated

[tomas-pajurek]

Currently, the blob metadata that are set via client methods are not validated. Azure Storage has relatively exotic restrictions on metadata keys:

https://learn.microsoft.com/en-us/rest/api/storageservices/setting-and-retrieving-properties-and-metadata-for-blob-resources

Therefore, it would be useful implement the restrictions also in this library. When doing so, checking to size limit of the metadata could be implemented as well.