Support adding named vectors to a collection

[bevzzz]

This PR:

• adds collection.config.addVector method to support creating new named vectors on the collection. Only named vectors can be created and existing named vectors cannot be modified.
  The method accepts the same arguments as collections.create(... { vectorizers: XXX }) with the exception that anonymous vectors are not permitted, e.g. vectorizer.none().
• fixes a bug whereby passing a vector with a custom name as a single object and passing it as an array [single object] to collections.create() would result in 2 different behaviors: custom name changed to default vs custom name preserved.

// Both cases should generate vectorConfig: { 'custom': {...} }
collections.create(..., { vectorizers: vectorizer.none('custom') });
collections.create(..., { vectorizers: [vectorizer.none('custom')] });

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca