Do not mutate data when using custom text format

[ahuang11]

I noticed that it previously mutated the data (e.g. added a label column when data['label'] = data.apply(lambda row: template_str.format(**row), axis=1)).

This PR fixes that:

import hvplot.pandas
import pandas as pd

df = pd.DataFrame(
    {
        "City": ["Buenos Aires", "Brasilia", "Santiago", "Bogota", "Caracas"],
        "Country": ["Argentina", "Brazil", "Chile", "Colombia", "Venezuela"],
        "Latitude": [-34.58, -15.78, -33.45, 4.60, 10.48],
        "Longitude": [-58.66, -47.91, -70.66, -74.08, -66.86],
    }
)
df.hvplot.points("Longitude", "Latitude", by="City") * df.hvplot.labels(
    "Longitude", "Latitude", text="City ({Latitude:.1f}N, {Longitude:.1f}E)", by="City"
)
df

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.41%. Comparing base (6c96c7e) to head (5a176c8).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1352      +/-   ##
==========================================
+ Coverage   87.39%   87.41%   +0.02%     
==========================================
  Files          50       50              
  Lines        7490     7502      +12     
==========================================
+ Hits         6546     6558      +12     
  Misses        944      944              

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[philippjfr]

I think this is the correct thing to do.

[maximlt]

Unrelated test failure.