feat: better customize header in step reports (through .get_step_report(header=...)

[rich-iannone]

This PR makes a second attempt to better let the user customize the header in a step report. Previously, the header= arg would accept text that would completely replace the header. However, the header in a step report consists of a title element and a larger details element. You couldn't replace only the title portion without wiping out the details part in the process.

Here, we allow for use of templating, where the default template is "{title}{details}" (and header=":default:" calls up this standard template). If you wanted to replace the title and keep the details, you'd supply "Custom Title {details}". Here's an example:

import pointblank as pb

validation = (
    pb.Validate(data=pb.load_dataset(dataset="game_revenue", tbl_type="polars"))
    .col_vals_lt(columns="item_revenue", value=200)
    .col_vals_gt(columns="item_revenue", value=0)
    .col_vals_gt(columns="session_duration", value=5)
    .col_vals_in_set(columns="item_type", set=["iap", "ad"])
    .col_vals_regex(columns="player_id", pattern=r"[A-Z]{12}\d{3}")
    .interrogate()
)

validation.get_step_report(
    i=3,
    header="Custom Title {details}",
)

Fixes: #111

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.14%. Comparing base (f09a8bb) to head (7fc14e3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #142   +/-   ##
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  Coverage   99.14%   99.14%           
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  Files          17       17           
  Lines        3721     3739   +18     
=======================================
+ Hits         3689     3707   +18     
  Misses         32       32           

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