Support string subdomain IDs

[pbrubeck]

No description provided.

[jorgensd]

Is there a particular use-case you have in mind for string domain ids?
In general, I think we should revisit the whole ufl.Measure implementation, as it seems a bit limiting.
Especially the integral_types being strings rather than a typing Protocol.
Whenever we add strings we add tons of undefined behavior (or undocumented behavior).

[pbrubeck]

Is there a particular use-case you have in mind for string domain ids?
In general, I think we should revisit the whole ufl.Measure implementation, as it seems a bit limiting.
Especially the integral_types being strings rather than a typing Protocol.
Whenever we add strings we add tons of undefined behavior (or undocumented behavior).

I can update docstrings. I'm fine if this is revisited. We should avoid breaking API.

firedrakeproject/firedrake#5199