[VSA] Tooltip setting for custom value stream

Release notes

Problem to solve

When you look at value stream analytics, it is hard to understand what each stream represents. For default value streams we present a tool tip explaining how this stream is calculated. This does not appear for custom streams and therefore, users do not know what this represents.

Proposal

When a user creates a new value stream, allow them to also insert text for the tool tip of this custom stream

similar to:

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.