Add ingest field access pattern to specification

[jbaiera]

Adds the new field access pattern to the specification. I didn't update any example code because this is not really expected to be a common option to set.

Relates to:

• Support flexible field access pattern in ingest pipelines elasticsearch#133270
• Add entry for ingest pipeline field access pattern feature docs-content#2994

Closes #5310

[pquentin]

Thanks! I opened #5310 to get CI results, but next time please open this from a branch instead of a fork. I realize that this is an annoying limitation, hopefully it will go away at some point.

[jbaiera]

but next time please open this from a branch instead of a fork

Ah, my apologies! Didn't realize that was a restriction. Will do in the future. Would you like me to reopen it as a branch?

[pquentin]

Thanks! LGTM.

No need to open from a branch as no new changes are planned.