feat: Add the send_slack_notification(), which creates a Slack notification action

[rich-iannone]

This PR adds a new send_slack_notification() function that enables sending validation results to Slack via webhooks. The main features here are:

• support for both step-level notifications and validation summaries (in one function)
• customizable message templates for nice message formatting
• debug mode for testing message formatting
• complete set of template variables for both step and summary messages
• error handling and dry-run capabilities

Here's an example of how it generally works:

import pointblank as pb

notify = pb.send_slack_notification(
    webhook_url="https://hooks.slack.com/services/...",
    step_msg="🚨 Step {step}: {failure_text}",
    summary_msg="📊 Summary: {n_failing_steps} of {n_steps} steps failed"
)

validation = (
    pb.Validate(data, 
        actions=pb.Actions(default=notify),
        final_actions=pb.FinalActions(notify)
    )
    .col_vals_gt(...)
    .interrogate()
)

validation

If the user should want to iterate on the template locally, the function can run at top level like this:

```{python}
import pointblank as pb

pb.send_slack_notification(
    step_msg="""🚨 *Validation Step Alert*
• Step Number: {step}
• Column: {column}
• Test Type: {type}
• Value Tested: {value}
• Severity: {level} (level {level_num})
• Brief: {autobrief}
• Details: {failure_text}
• Time: {time}""",
    summary_msg="""📊 *Validation Summary Report*
*Overview*
• Status: {highest_severity}
• All Passed: {all_passed}
• Total Steps: {n_steps}

*Step Results*
• Passing Steps: {n_passing_steps}
• Failing Steps: {n_failing_steps}
• Warning Level: {n_warning_steps}
• Error Level: {n_error_steps}
• Critical Level: {n_critical_steps}

*Table Info*
• Table Name: {tbl_name}
• Row Count: {tbl_row_count}
• Column Count: {tbl_column_count}

*Timing*
• Duration: {validation_duration}s
• Completed: {time}""",
    debug=True
)

Note that webhook_url= is unset (None) and debug=True.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.13%. Comparing base (f58865f) to head (a2e28d1).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #147   +/-   ##
=======================================
  Coverage   99.13%   99.13%           
=======================================
  Files          17       18    +1     
  Lines        3809     3828   +19     
=======================================
+ Hits         3776     3795   +19     
  Misses         33       33           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.