storage: move ExternalIODir into StorageConfig

[andrewbaptist]

Previously the ExternalIODir was stored as a variable inside the
BaseConfig struct and updated based on the StoreSpecs if it wasn't
explicitly set. This commit moves it into the StorageConfig and removes
it from the BaseConfig.

Epic: CRDB-41111

Release note: None

[cockroach-teamcity]

This change is 

[blathers-crl]

Your pull request contains more than 1000 changes. It is strongly encouraged to split big PRs into smaller chunks.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[andrewbaptist]

@RaduBerinde FYI - I'm investigating the failure in TestChangefeedExecLocality related to the change in ServerArgsPerNode.

[andrewbaptist]

The failure in TestChangefeedExecLocality appears to be a random flake under stress+race and is unrelated to my change (other than by touching the file it caused CI to stress+race this file). Since it was a timeout rather than anything actionable, I'm going to ignore it for this PR's perspective.

[RaduBerinde]

I am not sure that ExternalIODir belongs in storage.. It is not directly related to the stores. @jbowens WDYT? This is the definition:

	// ExternalIODir is the local file path under which remotely-initiated
	// operations that can specify node-local I/O paths (such as BACKUP, RESTORE
	// or IMPORT) can access files.

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @angles-n-daemons, @golgeek, @herkolategan, @jbowens, @michae2, @msbutler, and @rharding6373)