[v14] Access Plugins: Support dynamic credential reloading

[strideynet]

This PR:

• Refactors the access plugin's creation of the Teleport client to a single location rather than repeating this for each client. Adding new functionality would've involved me adding this at each of the client creation points and this seemed a bit silly to me.
• Adds a new configuration option to enable identity file reloading and to configure the interval

Before merging, wait for the Teleport Plugins PR: gravitational/teleport-plugins#956

changelog: Introduces support for configuring Access Plugins to automatically refresh the identity file from disk. This improves compatibility with Machine ID as a credential source.

Backports #32974