batches: Changeset Spec Exceeds Database Limit

Created by: BolajiOlajide

Closes #39242 (closed)

Test plan

Before #39471, we ran into an edge case where a batch change that generated a large changeset couldn't be saved in the database. The error gotten was

ERROR: string too long to represent as jsonb string (SQLSTATE 54000)

This happened because the size limit for the jsonb field in the changeset_specs table was around 256MB, so running a batch spec (example attached below) would return an error.

# Generates 676.3 MB changeset spec
name: large-changeset
description: Add Hello World to READMEs

on:
  - repositoriesMatchingQuery: file:README.md repo:^github.com/odoo/odoo$

steps:
  - run: find . -name "*.po" -exec sed -i 's/b/c/g' {} \;
    container: alpine:3

changesetTemplate:
  title: Hello World
  body: My first batch change!
  branch: hello-world-1 # Push the commit to this branch.
  commit:
    message: Append Hello World to all README.md files
  published: false # Do not publish any changes to the code hosts yet

This PR adds a check to inform the user to make use of transformChanges if the size of the changeset gets too large. Since #39471, that limit has increased to 1G.