[FEATURE]New fieldsummary PPL command

[YANG-DB]

Describe the solution you'd like
We propose adding a new fieldsummary command to OpenSearch PPL that would provide summary statistics for all fields in the current result set.

This command should:

1. Calculate basic statistics for each field (count, distinct count, min, max, avg for numeric fields)
2. Determine the data type of each field
3. Show the most frequent values and their counts for each field
4. Calculate the percentage of events that contain each field

Additionally, the command should support the following key optional parameters:

1. includefields:
   Specify which fields to include in the summary (e.g., | fieldsummary includefields="status_code,user_id,response_time")
2. excludefields:
   Specify which fields to exclude from the summary (e.g., | fieldsummary excludefields="internal_id,debug_info")
3. topvalues:
   Set the number of top values to display for each field (e.g., | fieldsummary topvalues=5)
4. maxfields:
   Limit the number of fields to display (e.g., | fieldsummary maxfields=20)
5. nulls:
   Include null/empty value counts (e.g., | fieldsummary nulls=true)

Example usage:

source = t
| where timestamp >= "2023-01-01" and timestamp < "2023-02-01"
| fieldsummary includefields="status_code,user_id,response_time" topvalues=3 nulls=true

This command would generate a table with summary statistics for the specified fields in the given date range, showing the top 3 values for each field and including null counts.

Example output:

Field	Count	Distinct	Min	Max	Avg	Type	Top Values	Nulls
status_code	10000	4	200	503	-	short	200 (8000, 80%) 404 (1500, 15%) 500 (400, 4%)	0
user_id	9500	1200	-	-	-	string	user123 (100, 1%) user456 (95, 1%) user789 (90, 0.9%)	500
response_time	10000	986	0.01	10.5	0.75	float	0.5 (2000, 20%) 0.75 (1800, 18%) 1.0 (1500, 15%)	0