Add sync setting parameter to Index

[jamescalam]

Should add sync setting to BaseIndex base class (with NotImplemented) and actual implementation to be added in PineconeIndex. Should (probably) incl:

• sync="error" — if local and remote not synchronized raise error
• sync="remote" — if local and remote not synchronized take remote as true (and update local to align)
• sync="local" — if local and remote not synchronized take local as true (and update remote to align)
• sync="merge" — if local and remote not synchronized merge both, adding routes from remote to local and vice-versa