feat: dynamic namespace parallelism. Fixes #14194

[isubasinghe]

Fixes #14194

Motivation

This change allows for more granular control of how namespace parallelism works.
This enables you to set parallelism at an individual namespace level instead of the more blanket namespace parallelism flag.

Modifications

This adds a namespace watcher that listens for changes in the namespace objects, by listening to a label. Whenever the labels workflows.argoproj.io/namespace-parallelism-limit is present and with a valid integer value (represented as string), we update the throttler with the new namespace limits.

Verification

An e2e test was added to verify that the code allows for dynamic namespace parallelism.
Additionally manual testing was performed to ensure non-happy paths work.

Documentation

Some documentation was added under parallelism to indicate the presence of this new feature. This could perhaps be more verbose with examples.