Allow timestamp attribute formatting + central timestamp formats

[spacegaier]

Breaking change

Proposed change

1. Centralize the definition of allowed timestamp formats rather than duplicating in many places.
2. Allow setting of format for attributes in entities card to be consistent with entity state rendering.

If the attribute is not a valid timestamp value, then the hui-timestamp-display will show "Invalid timestamp".

Before:

After:

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

type: entities
entities:
  - type: attribute
    entity: sun.sun
    attribute: next_rising
    format: datetime
    secondary_info: last-changed
    name: Next sun rising
  - entity: sun.sun
    secondary_info: last-changed
state_color: true

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

[bramkragten]

Line 49 https://github.com/home-assistant/frontend/pull/8162/files#diff-a13b3324902aa8c7b840e2d4d5070732e46824007d67d1eaac62341ef756e92aR49 should be moved below the check for stateObj