[YSQL] Add Json-compatible histogram to YsqlStatementStat

[acuskev]

Jira Link: DB-6803

Description

#14445 added the yb_latency_histogram column to the pg_stat_statements view. We want to extend this functionality to the output of SQL server / statements. Currently the output resembles:
{ "statements": [ { "query_id": 6810743978840536510, "query": "create table t1 as select i from generate_series(1, 1000) i", "calls": 1, "total_time": 169.233163, "min_time": 169.233163, "max_time": 169.233163, "mean_time": 169.233163, "stddev_time": 0.0, "rows": 0 }, { "query_id": -3987609402916575485, "query": "create table t3 as select i from generate_series(1, 100000) i", "calls": 1, "total_time": 1334.519109, "min_time": 1334.519109, "max_time": 1334.519109, "mean_time": 1334.519109, "stddev_time": 0.0, "rows": 0 }, ... ] }
This task will add a histogram field to YsqlStatementStat. This histogram field will then be written to Json and returned like the other fields in the above output, one histogram for each list entry. This task may require changes in pg_stat_statements.c and pgsql_webserver_wrapper.cc to convert the histogram struct to string or Json, and handle writing the new field.

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