Make ExternalInfo lsid text property of unbounded length

[sbesson]

This property is currently used in Glencoe's OMERO Zarr PixelBuffer implementation to store either the local path or the S3 URI to a multiscales group in a Zarr dataset. In the case of Zarr data stored on object store, the full URI can exceed the current 255 character limit.

In the same spirit as ome/openmicroscopy#4882, this property could be relaxed to be of unbounded length and the DB upgrade script for the next major version of OMERO could include an ALTER statement of type ALTER TABLE externalinfo ALTER COLUMN lsid TYPE TEXT;