feat: add text of brief to validation report table; enable more templating features for brief=

[rich-iannone]

This PR makes it so the text of the brief= parameter is displayed in the validation report table. We also make the brief= arg more flexible: True yields the automatically-generated brief (the 'autobrief') and we could use a few more templating params (e.g., "{auto}") when adding the brief= as a string.

Here's an example of how this works (and how it looks):

import pointblank as pb

validation = (
    pb.Validate(
        data=pb.load_dataset(dataset="small_table", tbl_type="polars"),
        tbl_name="small_table"
    )
    .col_vals_gt(columns="d", value=1000)
    .col_vals_le(columns="c", value=5, brief=True)
    .col_exists(columns=["date", "date_time"], brief="**A Brief**: {auto}")
    .interrogate()
)

validation

Fixes: #125

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.04%. Comparing base (2a69672) to head (ae4827a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #127   +/-   ##
=======================================
  Coverage   99.03%   99.04%           
=======================================
  Files          17       17           
  Lines        3619     3648   +29     
=======================================
+ Hits         3584     3613   +29     
  Misses         35       35           

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