Add custom serde support

[hypergonial]

Add support for custom serializers, as users (such as myself) may have previously used different formats to serialize & deserialize their custom_id and may want their components to continue working with flare.

This PR also adds a version char as the first character of the custom_id, so different serializers can be prevented from deserializing a different format they may not be expected to handle. Note that using serde versioning is optional and if a custom serializer is created with the version character set to None, then version checking will be disabled.

Note: I haven't tested if any of this works.