Add support for field descriptions in config flows

[balloob]

Breaking change

Proposed change

Allow defining descriptions for specific fields in data entry flows.

Marking as draft until we're happy about frontend: home-assistant/frontend#12119

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[probot-home-assistant]

Hey there @afaucogney, mind taking a look at this pull request as it has been labeled with an integration (derivative) you are listed as a code owner for? Thanks!
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[emontnemery]

Should we also check that the keys in "data_description" are present in "data"?