Separate out strategies to a separate repo

[CasperWA]

Following some reflections in the OP of #337, I think it may be nice to split out the strategies to a separate repository (e.g., oteapi-core-strategies or something).

The main benefits:

• Separate of concerns.
• The strategies can still be installed as part of oteapi-core through an extra, e.g., oteapi-core[strategies] or similar.
• It will greatly reduce the number of (unnecessary) dependencies, having plugins only install sub-dependencies that are actually necessary to use OTEAPI (see the full list in the updated pyproject.toml file in Sort dependencies #337.)

The main arguments against (that I can think of):

• Doubles the maintenance.
• Logical "usage" tests with strategies possibly becomes harder for this repository.
• All packages currently depending on oteapi-core will immediately lose the strategies, until they update their dependency specification to include the strategies extra (or similar) or explicitly also depend on the separated out core strategies package.