This PR brings a massive refactor/rewrite of the cache layer, making its handling of entities generic. This means that it becomes way easier to add more entities, easier to improve caching techniques, easier to manage memory, and avoid sticky code in duplicated code.

Major changes include:

• repo: reduced interface for full-text indexing, avoid the direct dependency with Bleve
• repo: introduce batched indexing, improve performance
• repo: return recognizable errors on "not found", to be able to distinguish from general errors
• repo: introduce support for muti-entities push/pull, and thus having to do only a single auth (fix git-bug push makes two git push #218, fix Bad output on 'git bug pull' #867)
• cache: introduce CachedEntityBase, base type for caching wrapper around an entity
• cache: introduce SubCache, generic cache handling for entities
• cache: introduce withSnapshot, generic efficient snapshot handling
• cache: emit BuildEvent when building the cache instead of printing, leaving full control for UIs to express those things in a meaningful way
• cache: each entity now has its own namespace in the cache: backend.Bugs().New(...) ... , reducing the functions names and making things cleaner

Generally, this untangle quite a lot of code and is a big step forward for git-bug.

Also, introduce ErrWaitGroup, a variant of sync.WaitGroup/errgroup.Group that takes func() error instead of func(), run all of them to completion (unless the go context expire), and assemble all the errors with the go1.20 multierror support. This IMHO should be included in the standard library.