Implement getSasToken for generating sas tokens for blobs in blob storage container

Description

A new method is required in BlobStore.java which allows us to generate SAS tokens for the blobs present in the blob storage container. The SAS tokens are used for modifying/creating/reading the blobs present inside the blob container. Permissions on the blob like read/write/create etc. can be provided along with the expiry time of the token.

Method signature is as below

public String getSasToken(final String dataPartitionId, final String filePath, final String containerName, final OffsetDateTime expiryTime, final BlobSasPermission permissions);