[FEATURE REQ] Get ETag after closing BlobOutputStream

**Is your feature request related to a problem? Please describe.**
When creating a Blob from getBlobOutputStream, I would like to get the ETag so that future conditional get blob requests work

**Describe the solution you'd like**
Add a single method to BlobOutputStream called getETag. This will fail before close is called and return a String ETag afterwards.

**Describe alternatives you've considered**
Client could call getProperties with the blob name. However, this is is racy.

**Additional context**
None

**Information Checklist**
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