[ServiceNow] need for sweeping improvements

[seanstory]

Problem Description

@codingogre has found a whole slew of issues in the ServiceNow connector, and shared the fixes he made on his fork. See: #3659

The task for this issue is to incorporate these into the mainline, but not necessarily by merging his exact changes.

Problems to address include:

• memory queue problems. Remove it, OR
  • configurable queue memory
  • decrease concurrent tasks
• pagination and fetching
  • increase default page size
  • reduce concurrent tables fetched
• configurable attachment handling
• improved logging and error handling
• add SSL cert validation and custom cert support
• Advanced Sync Rules rework to address scale issues

Proposed Solution

For each of the above, consult Shawn's approach, adapt it to our standards, add tests. If the diff is substantial, create its own PR.

By the end, we should be able to certify that the ServiceNow connector has significantly improved first-touch behaviors for large customers.

Additional Context

Slack thread: https://elastic.slack.com/archives/C07E6J58STD/p1751544217496509
SDH: https://github.com/elastic/sdh-search/issues/1671