feat: add User-Agent header

[ricellis]

Checklist

• Added tests for code changes or test/build only changes
• Updated the change log file (CHANGES.md|CHANGELOG.md) or test/build only changes
• Completed the PR template below:

Description

Add User-Agent header to requests.

Fixes #116

Approach

• Update build.gradle to add com/ibm/cloud/cloudant/spring/boot/ package section with Implementation-Version property
• Update CloudantFactory to construct and set the User-Agent header reading the version number from the com.ibm.cloud.cloudant.spring.boot package metadata.
• Added src/main/java/com/ibm/cloud/cloudant/spring/boot/package-info.java (not sure this is strictly necessary, but it doesn't hurt either).
• Update sample/build.gradle to correct issues observed when testing (repo ordering and Java compatibility)
• Update secrets baseline for line number changes

Schema & API Changes

• "No change"

Security and Privacy

• "No change"

Testing

• Added new tests:
  • src/test/java/unit/CloudantFactoryTest.java#testUserAgent()

Monitoring and Logging

• "No change"

[mojito317]

In the test we trust the System.getProperty methods (i.e. check the UA value with almost itself). I was wondering on a test case where System.getProperty methods are mocked, so this test would be more valuable. Other than that this looks good to me.