lsdb-server cannot filter based on string columns

[gitosaurus]

Bug report

Back-end filtering works for numeric quantities but not for string/enum quantities. Both of these attempts fail:

gaia3 = lsdb.read_hats(
    "http://epyc.astro.washington.edu:43210/hats/gaia_dr3/gaia/",
    filters=[
                ["phot_variable_flag", "=", "CONSTANT"],  # bare string
            ],
)
gaia3 = lsdb.read_hats(
    "http://epyc.astro.washington.edu:43210/hats/gaia_dr3/gaia/",
    filters=[
                ["phot_variable_flag", "=", '"CONSTANT"'],  # forced string literal
            ],
)

Both of these cause this back-end error from lsdb-server:

called `Result::unwrap()` on an `Err` value: NotYetImplemented("Data type Utf8 not yet implemented")

The first approach works with https://data.lsdb.io/hats/gaia_dr3/gaia, and the second does not (and perhaps should not).

NOTE: there are no columns in Gaia DR3 where phot_variable_flag has the value CONSTANT. There is only NOT_AVAILABLE and VARIABLE; be aware of that when testing.

Before submitting
Please check the following:

• I have described the situation in which the bug arose, including what code was executed, information about my environment, and any applicable data others will need to reproduce the problem.
• I have included available evidence of the unexpected behavior (including error messages, screenshots, and/or plots) as well as a description of what I expected instead.
• If I have a solution in mind, I have provided an explanation and/or pseudocode and/or task list.

[hombit]

Let's transfer this issue to https://github.com/astronomy-commons/lsdb-server ?