Add support for uniform sharding on instrumentation tests

[FrancescoRigoni]

Checklist

• [x] I've read and followed the Contribution Guidelines
• [x] step.yml and README.md is updated with the changes (if needed)

Version

Requires a MINOR version update

Context

In order to speed up our builds we have to take advantage of uniform test sharding when running UI tests. This step did not support it so I added it.

Changes

• Add configuration options for uniform sharding
• Add the sharding configuration (if needed) when invoking firebase test lab