Added parameter to style the container with error state

[elimm]

Added ability to set custom container decoration in case validation fails. It works with MultiSelectBottomSheetField and MultiSelectDialogField.

[maxbeech]

Thanks so much for this @elimm.
Can we please merge this? Using fields from this package really stands out at the moment without this as it's atypical for this not to be supported.

[elimm]

@CHB61 hasn't responded for a while, you can use my fork as an option

[maxbeech]

Will do, thank you!