Since it's possible to mix and match sequence_as_of and
tree_as_of dates in cladetime, sequences and reference
trees may have different ncov_metadata attributes
(dataset version, nexclade cli version, for example)
Add an ncov_metadata property to Tree that reflects
metadata for the tree_as_of date (as opposed to
CladeTime's ncov_metadata property, which reflects
sequence_as_of).

We'll use this new property to make sure we're using
the correct nextclade dataset when assigning
clades.

Still in the NCBI mindset, earlier versions of sequence.filter
used accession numbers to compare .fasta records to a set
of sequence "ids". However, for the processed Nextstrain
sequences, we need to use the "strain" column

We will need to instantiate a Tree object from CladeTime
when assigning clade sequences. Thus, we shouldn't use
CladeTime objects to do this because circulate dependencies

Adding these parameters allows additional filtering on
sequence metadata for min and max collection dates. This
is in support of clade assignemnts, where we'll only
want to assign clades to a small subset of sequences based
on their collection date. Behavior is unchanged if these
new parameters aren't specified.

This will allow re-use of that function when working with
collection begin/end dates in sequence assignment

Additional test cases for date commit

This new method is how clade time users (including people
using the upcoming CLI) will do custom clade assignments.
After validating dates, assign_clades calls out to existing
functions, performing a kind of "mini pipeline" to return
a LazyFrame with the results from Nextclade merged with
metdata from the sequences being assigned.

This changeset represents new tests for the assign_clades
method, as well as updates that reflect some refactoring
that occurred along the way.

This changeset returns a summarized version of the clade
assignments as well as some metadata about the clade
assignment process.