Description of changes:
Update user agent string to show the correct DDBEC library version. Previously, we had a) never updated the version string of our library from 1.0, and b) were needlessly including the version of the AWS Java SDK (this is already added as part of the user agent string by the SDK).

Rather than adding another place we have to hardcode the correct version, I'm pulling it from pom.xml.

I've also updated the format of our appended string to more closely match what we do in Python DDBEC and Java ESDK.

Testing:
This gives us a full user agent string like the following:

aws-sdk-java/1.11.460 Mac_OS_X/10.15.7 OpenJDK_64-Bit_Server_VM/11.0.5+10-b520.38 java/11.0.5 DynamodbEncryptionSdkJava/2.0.1

(First section is generated by AWS SDK, last section is ours)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.