Allow roles to have an optional description

[legrego]

Role names do not always convey the purpose or scope of a role. In order to improve the administrative experience, we should allow roles to have an optional description.

Role descriptions are available in both the stateful and serverless offering. The requirements do not differ between these distribution types.

Requirements

• The Create Role UI should allow a role to be created with a description.
• The Create Role API should allow a role to be created with a description.
• Role descriptions should be updatable via both UI and API.
• Validation for the length and content of descriptions shall be handled by ES, with errors returned to the UI/API appropriately.
• The Role Listing Page should display the description of each role, truncated if necessary.
• The Get Role API should include the description in its response
• API documentation should be updated accordingly
• API changes must be backwards-compatible. A role should be creatable/updatable without a description.
• The roles combo box that we render on the User and Role Mappings page should be updated to include the description in some capacity (e.g. tooltip or similar)

Mockup:

Changes from mockup:

• Remove the field description which states This description will be visible when assigning roles from the Organization page.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[legrego]

This is unblocked now that elastic/elasticsearch#107088 has merged.