Allow custom endpoint descriptions

[gramian]

Dear Developers,

when using a http_server input to register an endpoint, then using Benthos' /endpoints endpoint all user registered endpoints have the description "Post a message into Benthos." (see: https://github.com/benthosdev/benthos/blob/main/internal/impl/io/input_http_server.go#L232 ). I would like to propose to add a description property to the http_server input so some custom meta information about an endpoint can be stored.

Thank you very much for considering this idea

[gramian]

If I may add, it would be superb if interpolation could be used for the proposed description property.